mfd: qnap-mcu: Add proper error handling for command errors

mfd: qnap-mcu: Add proper error handling for command errors

Further investigation revealed that the MCU in QNAP devices may return
two error states. One "@8" for a checksum error in the submitted command
and one "@9" for any generic (and sadly unspecified) error.

These error codes with 2 data character can of course also be shorter
then the expected reply length for the submitted command, so we'll
need to check the received data for error codes and exit the receive
portion early in that case.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/20251113165218.449616-5-heiko@sntech.de
Signed-off-by: Lee Jones <lee@kernel.org>