mmc: dw_mmc-pltfm: remove static from dw_mci_pltfm_remove

dw_mci_pltfm_remove gets exported and used by dw_mmc-exynos, so should
not be static.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>