staging: mt7621-mmc: Fix different caps for different devices

Current code saves the need for polling in a global variable, that
leeds to problems, when the driver should handle multiple devices with
different capabilities. By looking up the capability in the mmc_host
capabilities, they now have per device scope.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>