gpio: pxa: remove set but not used variable 'gpio_offset'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpio/gpio-pxa.c: In function "pxa_gpio_probe":
drivers/gpio/gpio-pxa.c:629:35: warning:
 variable "gpio_offset" set but not used [-Wunused-but-set-variable]
  int irq0 = 0, irq1 = 0, irq_mux, gpio_offset = 0;
                                   ^
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>