staging: usbip: set usbipd server port via "-t" / "--tcp-port" option.

Add an option "-t" / "--tcp-port" to specify the TCP port to listen
on.  Downcase associated variables as they're no longer constants.

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>