It is sometimes necessary to give up on trying to claim the host lock,
especially if that happens in a thread that has to be stopped.

While at it, fix the description for mmc_claim_host() which was wrong.
Signed-off-by: NNicolas Pitre <npitre@mvista.com>
Signed-off-by: NPierre Ossman <drzeus@drzeus.cx>