spi: rzv2h-rspi: Fix DMA transfer error handling for signal interruption

spi: rzv2h-rspi: Fix DMA transfer error handling for signal interruption

wait_event_interruptible_timeout() can return a negative error code when
interrupted by a signal. The original code treated all non-zero return
values as success, which would incorrectly synchronize DMA channels and
return 0 instead of propagating the interruption error.

Fixes: fa08b566860b ("spi: rzv2h-rspi: add support for DMA mode")
Signed-off-by: Felix Gu <ustc.gu@gmail.com>
Reviewed-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Tested-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://patch.msgid.link/20260627-rspi-v1-1-170c93ee14da@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>