mmc: sdio: Allow sdio operations in other threads during sdio_add_func()

This fixes a bug introduced by 807e8e40

 ("mmc: Fix sd/sdio/mmc
initialization frequency retries") that prevented SDIO drivers from
performing SDIO commands in their probe routines -- the above patch
called mmc_claim_host() before sdio_add_func(), which causes a deadlock
if an external SDIO driver calls sdio_claim_host().

Fix tested on an OLPC XO-1.75 with libertas on SDIO.

Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
Reviewed-and-Tested-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Chris Ball <cjb@laptop.org>