gpio: pcf857x: get rid of slock spinlock

The spinlock 'slock' is used now to protect pcf857x_irq() from itself
which is unnecessary (especially after switching to use threaded
IRQs). Hence, remove it and use mutex to protect device data in IRQ
handler.

Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>