gpio: pcf857x: fixup smatch WARNING

gpio: pcf857x: fixup smatch WARNING

6e20a0a429bd4dc07d6de16d9c247270e04e4aa0
(gpio: pcf857x: enable gpio_to_irq() support)
added new smatch warnings

drivers/gpio/gpio-pcf857x.c:288 pcf857x_probe() error: we previously \
	assumed 'pdata' could be null (see line 277)
drivers/gpio/gpio-pcf857x.c:364 pcf857x_probe() warn: variable dereferenced\
	 before check 'pdata' (see line 292)
drivers/gpio/gpio-pcf857x.c:421 pcf857x_remove() error: we previously\
	 assumed 'pdata' could be null (see line 410)

This patch fixes it

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>