pinctrl: iterate over u300_pmx_mask's in u300_pmx_endisable

Fix u300_pmx_endisable() to iterate over the list of 'bits' and
'mask' populated as part of u300_pmx_functions.mask[]

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>