ath6kl: propagate errors on hifEnableFunc() failures

This will be passed along to the SDIO probe routine when
it fails. This will generate better error code to the user
when loading the module if it fails.

Cc: Naveen Singh <nsingh@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>