mfd: qnap-mcu: Handle errors returned from qnap_mcu_write

qnap_mcu_write can return errors and those were not checked before.
So do that now.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250804130726.3180806-3-heiko@sntech.de


Signed-off-by: Lee Jones <lee@kernel.org>