mfd: qnap-mcu: Convert to guard(mutex) in qnap_mcu_exec

guard() makes sure that the mutex gets unlocked when the function returns
and thus removes the need for unlock gotos or similar mechanisms and
therefore allows for a simpler function structure.

So convert the qnap_mcu_exec function to use it.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250804130726.3180806-4-heiko@sntech.de


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