Staging: sxg: Fix leaks and checksum errors in transmit code path

Fix the transmit function for the following:
* Free XmtCmd in the error code path. This use to leak memory in
  error conditions.
* Do pci mapping after the checksum operations are over. They can
  reallocate the skb at a different location.
* Fix UDP checksum errors which were seen in wireshark

Signed-off-by: LinSysSoft Sahara Team <saharaproj@linsyssoft.com>
Signed-off-by: Mithlesh Thukral <mithlesh@linsyssoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>