usbip: stub_rx: fix static checker warning on unnecessary checks

Fix the following static checker warnings:

The patch c6688ef9: "usbip: fix stub_rx: harden CMD_SUBMIT path
to handle malicious input" from Dec 7, 2017, leads to the following
static checker warning:

    drivers/usb/usbip/stub_rx.c:346 get_pipe()
    warn: impossible condition
'(pdu->u.cmd_submit.transfer_buffer_length > ((~0 >> 1))) =>
(s32min-s32max > s32max)'
    drivers/usb/usbip/stub_rx.c:486 stub_recv_cmd_submit()
    warn: always true condition
'(pdu->u.cmd_submit.transfer_buffer_length <= ((~0 >> 1))) =>
(s32min-s32max <= s32max)'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>