can: sja1000: Validate initialization state in start method

When sja1000 is not compiled as module the SJA1000 chip is only
initialized during device registration on kernel boot. Should the chip
get a hardware reset there is no way to reinitialize it without re-
booting the Linux kernel.

This patch adds a check in sja1000_start if the chip is initialized, if
not we initialize it.

Signed-off-by: Mirza Krak <mirza.krak@hostmobility.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>