mmc: cd-gpio: protect against NULL context in mmc_cd_gpio_free()

Do not oops, even if mmc_cd_gpio_free() is mistakenly called on a driver
cleanup path, even though a previous call to mmc_cd_gpio_request() failed.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
[stable@: please apply to 3.3-stable]
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>