SM501: Check SM501 ID register on initialisation

When binding the driver, check the ID register for a valid identity, in case
the SM501 is not functioning correctly.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>