vmxnet3: make descriptor count checks more robust

When we update credits there is a potential for a race causing an
overflow of vxcr_next (i.e. incrementing it past vxcr_ndesc). Change the
check to >= rather than == to be more robust against this.

Reviewed by: emaste
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D43712