USB: musb: clear the Blackfin interrupt pending bits early in the ISR

If we clear the interrupt pending bits at the end, we sometimes return too
fast and have the same interrupt assert itself.  There is no way in a
Blackfin system to force a sync of this state, so the hardware manual
instructs people to clear interrupt flags early in their ISR.

Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>