platform/chrome: cros_ec_lpc: MEC access can return error code

platform/chrome: cros_ec_lpc: MEC access can return error code

cros_ec_lpc_io_bytes_mec was returning a u8 checksum of all bytes
read/written, which didn't leave room to indicate errors. Change this
u8 to an int where negative values indicate an error, and non-negative
values are the checksum as before.

Tested-by: Dustin L. Howett <dustin@howett.net>
Signed-off-by: Ben Walsh <ben@jubnut.com>
Link: https://lore.kernel.org/r/20240605063351.14836-2-ben@jubnut.com
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>