serial: max3100: Replace open-coded parity calculation with parity8()

Refactor parity calculations to use the standard parity8() helper.
This change eliminates redundant implementations.

Co-developed-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Link: https://lore.kernel.org/r/20250515081311.775559-1-visitorckw@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>