mmc: dw_mmc: use __devexit_p macro for .remove()

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Girish K S <girish.shivananjappa@linaro.org>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>