can: mcp251x: fix endless loop in interrupt handler if CANINTF_MERRF is set

Commit d3cd15657516141adce387810be8cb377baf020e introduced a bug, the
interrupt handler would loop endlessly if the CANINTF_MERRF bit is set,
because it's not cleared.

This patch fixes the problem by masking out the CANINTF_MERRF and all other
non interesting bits.

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