Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/can/ti_hecc.c: In function 'ti_hecc_mailbox_read':
drivers/net/can/ti_hecc.c:533:12: warning:
 variable 'mbx_mask' set but not used [-Wunused-but-set-variable]

It is never used so can be removed.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Jeroen Hofstee <jhofstee@victronenergy.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>