can: flexcan: factor out enabling and disabling of interrupts into separate function

The upcoming patches are going to move the enabling and disabling of the
interrupts. Introduce convenience functions to make these patches simpler.

Link: https://lore.kernel.org/r/20201119100917.3013281-2-mkl@pengutronix.de


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