mrf24j40: separate h/w init and add checkings

separate the mrf24j40 hardware initialisation from probe()
and adds the sanity checkings.

These checkings are required if somebody hasn't a right spi configuration
the probe function should fail. So we have to return from there.

Signed-off-by: Varka Bhadram <varkab@cdac.in>
Signed-off-by: David S. Miller <davem@davemloft.net>