be2net: Refactor be_xmit_enqueue() routine

- Reduce code duplication by moving WRB-frags setup into a function.
- Do not setup WRB-header before frags are setup, which is unncessary if
  there's errors while setting up frags. We should only grab an entry for
  the header, setup the frags and if everything is fine setup the header.
- The error cleanup can be moved into a small function.

Signed-off-by: Sriharsha Basavapatna <sriharsha.basavapatna@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>