can: at91_can: make can_id of mailbox 0 configurable

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 an unused identifier.

This patch implements the second part of the workaround. A sysfs entry
"mb0_id" is introduced. While the interface is down it can be used to
configure the can_id of mailbox 0. The default value id 0x7ff.

In order to use an extended can_id add the CAN_EFF_FLAG (0x80000000U)
to the can_id. Example:

- standard id 0x7ff:
echo 0x7ff      > /sys/class/net/can0/mb0_id

- extended id 0x1fffffff:
echo 0x9fffffff > /sys/class/net/can0/mb0_id

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>
For the Documentation-part:
Acked-by: Wolfram Sang <w.sang@pengutronix.de>