fix recent MMC driver compile breakage

The OMAP MMC driver previously supported multiple cards per MMC host,
but upper-layer support for this was recently removed (commit
b855885e3b60cf6f9452848712a62517b94583eb).  So, remove this support from
this driver, and add a missing include.

Compile-tested only.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>