can: SJA1000 add missing spin_lock_init()

As remarked by Sam Ravnborg the spin_lock variable, that has been introduced
in commit 57c8a456640fa3ca777652f11f2db4179a3e66b6 ("can: Fix SJA1000 command
register writes on SMP systems") has not been initialized properly.

This patch adds the initialization to allow spinlock debugging.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
CC: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>