ks8851: Fix missing mutex_lock/unlock

Move the ks8851_rdreg16 call above the call to request_irq and cache
the result for subsequent repeated use.  A spurious interrupt may
otherwise cause a crash.  Thanks to Stephen Boyd, Flavio Leitner, and
Ben Hutchings for feedback.

Signed-off-by: Matt Renzelmann <mjr@cs.wisc.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>