Fix ym2610 reentrant write to port A (part 2)

Follow up fix to f5e2268, in
the interrupt handler, the driver should not recapture the
register context when writing to the YM2610's port A.

Author: dciabrin

nullsound/timer.s

@@ -86,9 +86,11 @@ update_timer_state_tracker::
         call    ym2610_wait_available
 
         ;; reset interrupt timer B
+        ;; YM2610 register context is only captured by regular code,
+        ;; i.e. outside of the interrupt handler
         ld      b, #REG_TIMER_FLAGS
         ld      c, #0x2a
-        call    ym2610_write_port_a
+        call    ym2610_write_port_a_no_ctx
 
         ;; step2: at this stage, if we interrupted a ym2610_write_port_a,
         ;; restore the YM2610 register context before returning from the

nullsound/ym2610.s

@@ -63,6 +63,25 @@ ym2610_write_port_a::
         pop     af
         ret
 
+;;; ym2610_write_port_a_no_ctx
+;;; --------------------------
+;;; IN:
+;;;    b: register address in ym2610
+;;;    c: data to set
+;;; (all registers are preserved)
+ym2610_write_port_a_no_ctx::
+        push    af
+        ;; select register address
+        ld      a, b
+        out     (PORT_YM2610_A_ADDR), a
+        call    _ym2610_wait_address_write
+        ;; set data in the selected register
+        ld      a, c
+        out     (PORT_YM2610_A_VALUE), a
+        call    _ym2610_wait_data_write
+        pop     af
+        ret
+
 ;;; ym2610_write_port_b
 ;;; -------------------
 ;;; IN: