LinuxKPI: 802.11: fix re-creation of VAP

After dbf7691999abe (as indicated in that commit message) EEXISTS
errors may be seen. From current investigation that is due to the
single-VAP support in LinuxKPI 802.11 and the related calls to
mo_start() and mo_stop() and them being disabled in ic_parent()
currently. Calling mo_stop() in lkpi_ic_vap_delete() seems to
fix the problem and new VAPs (wlan interfaces) can be created again
after being destroyed.

Fixes: dbf7691999abe ("improve lkpi_ic_vap_delete()")
Sponsored by: The FreeBSD Foundation
MFC after: 3 days