pinctrl: single: Fix build error

If pcs->is_pinconf is false, it means does not support pinconf.
If pcs->is_pinconf is true, is_generic flag is always true.

This patch fixes below build error:

  CC [M]  drivers/pinctrl/pinctrl-single.o
drivers/pinctrl/pinctrl-single.c: In function 'pcs_probe':
drivers/pinctrl/pinctrl-single.c:1441:3: error: assignment of member 'is_generic' in read-only object
make[2]: *** [drivers/pinctrl/pinctrl-single.o] Error 1
make[1]: *** [drivers/pinctrl] Error 2
make: *** [drivers] Error 2

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>