can: bfin_can: declare locally used functions static

This patch fixes the following sparse warning:

drivers/net/can/bfin_can.c:415:13: warning: symbol 'bfin_can_interrupt' was not declared. Should it be static?
drivers/net/can/bfin_can.c:507:19: warning: symbol 'alloc_bfin_candev' was not declared. Should it be static?

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>