can: flexcan: give member of flexcan_priv holding mailboxes a sensible name

This patch gives the member of flexcan_priv holding mailboxes a sensible name,
by renaming from "cantxfg" to "mb":

    struct flexcan_priv::cantxfg -> struct flexcan_priv::mb

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>