can: at91_can: clean up usage of AT91_MB_RX_FIRST and AT91_MB_RX_NUM

This patch cleans up the usage of two macros which specify the mailbox
usage. AT91_MB_RX_FIRST and AT91_MB_RX_NUM define the first and the
number of RX mailboxes. The current driver uses these variables in an
unclean way; assuming that AT91_MB_RX_FIRST is 0;

This patch cleans up the usage of these macros, no longer assuming
AT91_MB_RX_FIRST == 0.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>