irda: irttp: allow zero byte packets

Sending zero byte packets is not neccessarily an error (AF_INET accepts it,
too), so just apply a shortcut. This was discovered because of a non-working
software with WINE. See

  http://bugs.winehq.org/show_bug.cgi?id=19397#c86
  http://thread.gmane.org/gmane.linux.irda.general/1643



for very detailed debugging information and a testcase. Kudos to Wolfgang for
those!

Reported-by: Wolfgang Schwotzer <wolfgang.schwotzer@gmx.net>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Mike Evans <mike.evans@cardolan.com>
Signed-off-by: David S. Miller <davem@davemloft.net>