usb: musb: remove an unused variable

After the only users of this variable got removed, we now get a
warning about 'otg' being unused:

drivers/usb/musb/da8xx.c: In function 'da8xx_musb_interrupt':
drivers/usb/musb/da8xx.c:226:19: error: unused variable 'otg' [-Werror=unused-variable]

Fixes: d2852f2d

 ("usb: musb: remove references to default_a of struct usb_otg")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>