can: c_can: Add and make use of 32-bit accesses functions

Add helpers for 32-bit accesses and replace open-coded 32-bit access
with calls to helpers. Minimum changes are done to the pci case, as I
don't have access to that hardware.

Tested-by: Thor Thayer <tthayer@altera.com>
Signed-off-by: Thor Thayer <tthayer@altera.com>
Signed-off-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>