pinctrl: intel: ensure error return ret is initialized

In the (unlikely) event that community->ngpps is zero, or if every
gpp->gpio_base is less than zero, then an ininitialized value in
ret is returned by function intel_gpio_add_pin_ranges. Fix this by
ensuring ret is initialized to zero.  It's a moot point, but I think
it is worthwhile ensuring this corner case is fixed.

Detected by CoverityScan, CID#1462415 ("Uninitialized scalar variable")

Fixes: a60eac32 ("pinctrl: intel: Allow custom GPIO base for pad groups")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>