sc16is7xx: enable the clock

Although clk_disable_unprepare() is called both on .remove()
and in .probe() error path, the clock is never actually enabled.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Jon Ringle <jringle@gridpoint.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>