can: at91_can: don't use mailbox 0

Due to a chip bug (errata 50.2.6.3 & 50.3.5.3 in
"AT91SAM9263 Preliminary 6249H-ATARM-27-Jul-09") the contents of mailbox
0 may be send under certain conditions (even if disabled or in rx mode).

The workaround in the errata suggests not to use the mailbox and load it
with a unused identifier.

This patch implements the first part of the workaround, it updates
AT91_MB_RX_NUM and AT91_MB_RX_FIRST (and the inline documentation)
so that mailbox 0 stays unused.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Acked-by: Kurt Van Dijck <kurt.van.dijck@eia.be>