can: flexcan: FLEXCAN_IFLAG_MB: add () around macro argument

This patch fixes the following checkpatch warning:

| Macro argument 'x' may be better as '(x)' to avoid precedence issues

Fixes: cbffaf7a ("can: flexcan: Always use last mailbox for TX")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>