xen-netfront: fix netfront create_dev error path

The failure path for allocating rx grant refs should not try to free tx
grant refs because tx grant refs were allocated after that. Also fix the
error path for xen_net_read_mac.

Submitted by: Wei Liu <wei.liu2@citrix.com>
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D3891
Sponsored by: Citrix Systems R&D