xen-netfront: fix initialization

A couple of mostly cosmetic fixes for the final initialization of netfront:

• Switch to "connected" state before starting to kick the rings.
• Correctly use "rxq" in the initialization loop (previously rxq was not updated in the loop, and netfront would kick np->rxq[N] several times).
• Declare and define xn_connect as static, it's not used outside of this file.

Reviewed by: Wei Liu <wei.liu2@citrix.com>
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D6657