mmc: Fix bus width setting in enhanced strobe mode

The 8-bit bus width needs to be set first before switching to
DDR bus width when entering HS400 in enhanced strobe mode.
Also use the mmc_select_bus_width() for doing this instead of
rewriting portion of that code.

Change-Id: If9bec799de77714d7183c812a0ba04a9a4ac48f5
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>