staging: gdm72xx: fix unused variable warning in gdm_usb_send

This patch fixes an unused variable warning in gdm_usb_send
(when CONFIG_WIMAX_GDM72XX_K_MODE=n), which was introduced in
commit 1a276b80 (staging:
gdm72xx: protect access of rx / tx structs).

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>