realtek_mst_i2c_spi.c: Don't depend on int overflows

Be explicit to mask the first byte after the shifts as
highlighted by Angel Pons.

BUG=none
BRANCH=none
TEST=builds

Change-Id: I7d1215678094d709e79b8f8c96aa3810586cd72e
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Spotted-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/48974
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Shiyu Sun <sshiyu@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed