mmc: dw_mmc: change the callback function name.

To prevent the confusion, use the get_mmc_clk() instead of mmc_clk().
get_mmc_clk() is more exactly name.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>