In case the network device has a RX or TX control queue, the correct number of TX/RX descriptors is contained
in the second entry of the isc_ntxd (or isc_nrxd) array, rather than in the first entry.
This case is correctly handled by iflib_device_register() and iflib_pseudo_register(), but not by
iflib_netmap_attach(). If the first entry is larger than the second, this can result in a panic.
This change fixes the bug by introducing two helper functions that also lead to some code simplification.
PR 247647 https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=247647