gpio: generic: fix signedness bug found by cppcheck

cppcheck reports this:

(style) int result is returned as long value. If the return
value is long to avoid loss of information, then you have
loss of information.

This can be fixed with (1UL << pin) but that is the same
as using <linux/bitops.h> that already use 1UL so take
this approach.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>