gpio: generic: clamp values from bgpio_get_set()

The bgpio_get_set() call should return a value clamped to [0,1],
the current code will return a negative value if reading
bit 31, which turns the value negative as this is a signed value
and thus gets interpreted as an error by the gpiolib core.
Found on the gpio-mxc but applies to any MMIO driver.

Cc: [e-mail] # 4.3+
Cc: [e-mail]
Cc: Vladimir Zapolskiy <[e-mail]>
Fixes: e20538b82f1f ("gpio: Propagate errors from chip->get()")
Reported-by: Clemens Gruber <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/gpio/gpio-generic.c

···
 	unsigned long pinmask = bgc->pin2mask(bgc, gpio);
 
 	if (bgc->dir & pinmask)
-		return bgc->read_reg(bgc->reg_set) & pinmask;
+		return !!(bgc->read_reg(bgc->reg_set) & pinmask);
 	else
-		return bgc->read_reg(bgc->reg_dat) & pinmask;
+		return !!(bgc->read_reg(bgc->reg_dat) & pinmask);
 }
 
 static int bgpio_get(struct gpio_chip *gc, unsigned int gpio)