mmc: make one-bit signed bitfields unsigned

Otherwise it can only take the values 0/-1 which doesn't seem to
have been intended.

drivers/mmc/host/sdhci.h:190:20: error: dubious one-bit signed bitfield

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Acked-by: Pierre Ossman <drzeus-list@drzeus.cx>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>