mmc: unconditionally define mmc_deinit()

We want to replace '#ifdef' by 'if (IS_ENABLED(CONFIG_...))' in our code.
Therefore functions should be defined unconditionally even if they are not
implemented.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>