gpio: generic: fix signedness bug found by cppcheck

cppcheck reports this:

(style) int result is returned as long value. If the return
value is long to avoid loss of information, then you have
loss of information.

This can be fixed with (1UL << pin) but that is the same
as using <linux/bitops.h> that already use 1UL so take
this approach.
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-generic.c

@@ -58,6 +58,7 @@ o        `                     ~~~~\___/~~~~    ` controller in FPGA is ,.`
 #include <linux/io.h>
 #include <linux/gpio.h>
 #include <linux/slab.h>
+#include <linux/bitops.h>
 #include <linux/platform_device.h>
 #include <linux/mod_devicetable.h>
 #include <linux/basic_mmio_gpio.h>
@@ -126,13 +127,13 @@ static unsigned long bgpio_read32be(void __iomem *reg)
 
 static unsigned long bgpio_pin2mask(struct bgpio_chip *bgc, unsigned int pin)
 {
-	return 1 << pin;
+	return BIT(pin);
 }
 
 static unsigned long bgpio_pin2mask_be(struct bgpio_chip *bgc,
 				       unsigned int pin)
 {
-	return 1 << (bgc->bits - 1 - pin);
+	return BIT(bgc->bits - 1 - pin);
 }
 
 static int bgpio_get_set(struct gpio_chip *gc, unsigned int gpio)