mmc: tmio: Cache interrupt masks

This avoids the need to look up the masks each time an interrupt is handled.
As suggested by Guennadi.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>