[MMC] constify mmc_host_ops

Let drivers constify MMC host method operations tables,
moving them from ".data" to ".rodata".

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>