can: sja1000_isa: convert to platform driver to support x86_64 systems

This driver is currently not supported on x86_64 systems because the
"isa_driver" interface is used (CONFIG_ISA=y). To overcome this
limitation, the driver is converted to a platform driver, similar to
the serial 8250 driver.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>