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.

Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
mmc: core: Add support for HS400

HS400 is a new bus speed mode introduced in eMMC5.0 specification. This
patch adds the capability in mmc core to switch to HS400 mode when both
the host and the card supports it.

Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
disable crc check