[MMC] pxamci: fix data timeout calculation

The MMC layer gives us two parts for the timeout calculation - a fixed
timeout in nanoseconds, and a card clock-speed dependent part.

The PXA MMC hardware allows for a timeout based on the fixed host clock
speed only.  This resulted in some cards being given a short timeout,
and therefore failing to work.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>