xen-netfront: rearrange error paths in setup_txqs

Coverity spotted double free errors in error path. Fix that by
removing the extraneous calls.

Coverity ID: 1349798
Submitted by: Wei Liu <wei.liu2@citrix.com>
Reviewed by: royger
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D5251