usb: gadget: langwell: drop langwell_otg support

Since there is no working (or even compilable) OTG_TRANSCEIVER support
for this driver, remove the dead code which depends on it at compile
time.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: stable@vger.kernel.org # v2.6.31+
Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Felipe Balbi <balbi@ti.com>