USB: musb: fix warnings in Blackfin regs

The recent commit "usb: musb: Add context save and restore support" added
some stubs for the Blackfin code so things would compile, but it also
added a bunch of warnings due to missing return statements.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>