USB: musb: Prevent multiple includes of musb.h

Add #ifndef to musb header file to prevent multiple inclusions.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>