mmc: at91_mci: reorder timer setup and mmc_add_host() call

As said in function comment mmc_add_host() requires that:
"The host must be prepared to start servicing requests
before this function completes."

During this function, at91_mci_request() can be invoqued
without timer beeing setup leading to a kernel Oops.
This has been reported inserting this driver as a module.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Reported-by: Wu Xuan <wux@landicorp.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>