staging: octeon-usb: remove unnecessary variable frame_number

Remove unnecessary local variable "frame_number" and its assignments from function
cvmx_usb_get_frame_numbe. Suggested by Coccinelle.

Signed-off-by: Himadri Pandya <himadri18.07@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>