USB: usbip: remove useless call in usbip_recv

Calling msg_data_left(&msg) is only useful for its return value,
which in this particular case is ignored.

Fix this by removing such call.

Addresses-Coverity-ID: 1427080
Fixes: 90120d15 ("usbip: prevent leaking socket pointer address in messages")
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>