mmc: core: Refactor bus speed selection code

The bus speed selection code in mmc_card_init() is convoluted, refactor
the code to simplify this and make it simpler for adding new bus speed
capabilities.

Change-Id: I1ee5365eaad42298a053d0b2d64b87b64bd05c5e
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>