MMC: OMAP: General cleanup for MMC multislot support

General code cleanup, modifications at some dev_* functions and
other hacks at mmc_omap_irq() for MMC multislot support.

Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Carlos Eduardo Aguiar <carlos.aguiar@indt.org.br>
Signed-off-by: Tony Lindgren <tony@atomide.com>