can: sja1000: fix define conflict on SH

Thias patch fixes a define conflict between the SH architecture and the sja1000
driver:

    drivers/net/can/sja1000/sja1000.h:59:0: warning:
        "REG_SR" redefined [enabled by default]
    arch/sh/include/asm/ptrace_32.h:25:0: note:
         this is the location of the previous definition

A SJA1000_ prefix is added to the offending sja1000 define only, to make a
minimal patch suited for stable. A later patch will add a SJA1000_ prefix to
all defines in sja1000.h.

Cc: linux-stable <stable@vger.kernel.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>