can: at91_can: fix reception of extended frames

The AT91_MID_MIDE bit must be set in order to receive extended frames.
The reception of an extended frame sets this bit, while reception of
standard frames resets it. This results in some lost extended frames in
an extended ID only environment. But leads to unpredictable lost
extended ID frames in a mixed environment.

The problem is fixed by setting the AT91_MID_MIDE after reception of a
CAN frame.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>