can: sja1000: use common prefix for all sja1000 defines

This is a follow up patch to:

    f901b6bc can: sja1000: fix define conflict on SH

That patch fixed a define conflict between the SH architecture and the sja1000
driver, by addind a prefix to one macro only. This patch consistently renames
the prefix of the SJA1000 controller registers from "REG_" to "SJA1000_".

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>