[MMC] sdhci: fix sdhci reset timeout

The reset register is automatically cleared when the reset has completed.
Hence, we should busy wait and not have a fixed delay.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>