hmt: adjust for new pwm_backlight->notify prototype

Commit cfc38999f (backlight: Pass device through notify callback)
added a struct device argument to the notify callback, but didn't
update the user of it in mach-hmt.c

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>