mfd: qnap-mcu: Calculate the checksum on the actual number of bytes received

mfd: qnap-mcu: Calculate the checksum on the actual number of bytes received

In the case of an error message, the number of received bytes can be
less than originally expected but still contain a valid message.

If the transfer itself ended in an error we would exit earlier already.

So calculate the checksum on the number of received bytes and not the
number of expected bytes.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/20251113165218.449616-2-heiko@sntech.de
Signed-off-by: Lee Jones <lee@kernel.org>