axi_jesd204_rx: Fixup write to up_cfg_buffer_delay

[gastmaier]

Change up_cfg_buffer_delay assignment from up_wdata [7:0] to [9:2].

PR Description

Test axi_jesd204_rx_regmap_tb.v was failing at register x90 with output:

Address 00000240, Expected 000103fc, Found 000103f0

The issue is at fields Buffer release delay [9:2] and Data path width alignment [1:0], the latter is always 2'b00.
Investigating jesd204_up_rx.v, I noticed that up_cfg_buffer_delay is assigned up_wdata[7:0] instead of [9:2], incoherent with the read instruction.
I ran all tests at tb and now all succeeds (xsim, 2023.1).

PR Type

• Bug fix (change that fixes an issue)
• New feature (change that adds new functionality)
• Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• I have followed the code style guidelines
• I have performed a self-review of changes
• I have compiled all hdl projects and libraries affected by this PR
• I have tested in hardware affected projects, at least on relevant boards
• I have commented my code, at least hard-to-understand parts
• I have signed off all commits from this PR
• I have updated the documentation (wiki pages, ReadMe files, Copyright etc)
• I have not introduced new Warnings/Critical Warnings on compilation
• I have added new hdl testbenches or updated existing ones

[gastmaier]

This change was introduced at #993 (direct link). looks like a mistake.
However there are more things in the regmap that I want to look into later, that can be added to this pr.

[gastmaier]

Looking into https://github.com/analogdevicesinc/linux/blob/main/drivers/iio/jesd204/axi_jesd204_rx.c#L53 (DWORD 90, BYTE 240)
Only bit 16 is writable with JESD204_RX_LINK_CONF2_BUFFER_EARLY_RELEASE at the subclass config.
Therefore, the up_cfg_buffer_delay (bits 9:2) are unused by the driver (same for no-OS.
On the HDL side, the write bits should match the read bits and the regmap
Updated the regmap to add the Data path width alignment as Reserved in the regmap (I followed the coding style of the current file to mitigate further changes).

Initially I expected to add more commits to this pr, but since it stayed untouched for so long, let's merge only this change.

[IstvanZsSzekely]

Tested, working