xen-netfront: remove pointless assignment in xn_ioctl

The variable error is assigned to 0 before entering the switch.
Assigning error to 0 before break pointless rewrites the real error
value that should be returned.

Coverity ID: 1304974
Submitted by: Wei Liu <wei.liu2@citrix.com>
Reviewed by: royger
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D5250