pinctrl: single: change function mask default value

The patch is inspired by more recent versions of the Linux driver.
Replacing the default value 0xffffffff of the function mask with 0 is
certainly more conservative in case the "pinctrl-single,function-mask"
DT property is missing.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Simon Glass <sjg@chromium.org>