smc911x: make smc911x_initialize return correct value

Make smc911x_initialize return -1 on error and number of interfaces
detected otherwise.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>