pinctrl: stmfx: Fix 'warn: unsigned <VAR> is never less than zero'

smatch warnings:
drivers/pinctrl/pinctrl-stmfx.c:225 stmfx_pinconf_get() warn: unsigned 'dir' is never less than zero.
drivers/pinctrl/pinctrl-stmfx.c:228 stmfx_pinconf_get() warn: unsigned 'type' is never less than zero.
drivers/pinctrl/pinctrl-stmfx.c:231 stmfx_pinconf_get() warn: unsigned 'pupd' is never less than zero.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>