usb: usbip: remove null check

The only caller of get_gadget_descs() has already dereferenced udc
before calling this function, so udc can not be NULL at this point of
the code and hence no use of checking it.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Reviewed-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>