gpio: mm-lantiq: Fix typo in the newly added header filename

The header with legacy struct of_mmio_gpio_chip and accompanying
APIs is called legacy-of-mm-gpiochip.h. Remove repetitive '.h'
at the end.

Fixes: a99cc668

 ("gpiolib: split of_mm_gpio_chip out of linux/of_gpio.h")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>