drm/amdgpu: Simplify RAS EEPROM checksum calculations

Rename update_table_header() to
write_table_header() as this function is actually
writing it to EEPROM.

Use kernel types; use u8 to carry around the
checksum, in order to take advantage of arithmetic
modulo 8-bits (256).

Tidy up to 80 columns.

When updating the checksum, just recalculate the
whole thing.

Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>