sm501: add inversion controls for VBIASEN and FPEN

Add flags to allow the driver to invert the sense of both VBIASEN and FPEN
signals comming from the SM501.

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>