can: ems_pci: Add read/write register and post irq functions

Add functions to read and write SJA1000 registers and also the
post irq routine

Signed-off-by: Gerhard Uttenthaler <uttenthaler@ems-wuensche.com>
Link: https://lore.kernel.org/all/20230120112616.6071-5-uttenthaler@ems-wuensche.com


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