mfd: qnap-mcu: Improve structure in qnap_mcu_exec

mfd: qnap-mcu: Improve structure in qnap_mcu_exec

Now with guard(mutex) in place, we can make the function's structure
a bit easier to read, by removing the nested if-else-clauses.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250804130726.3180806-5-heiko@sntech.de
Signed-off-by: Lee Jones <lee@kernel.org>