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

commit 0e9f480bb553d39ee06ccd45639ba7a5446a7b81 upstream.

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]
Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>