rtwn: narrow the epoch area

Rather than placing the epoch around the entire receive loop which
might call into rtwn_rx_frame() and USB and sleep, split the loop
into two[1] and leave us with one unlock/lock cycle as well.

PR: 249925
Reported by: thj, (rkoberman gmail.com)
Tested by: thj
Suggested by: adrian [1]
Reviewed by: adrian
MFC after: 3 days
Sponsored by: The FreeBSD Foundation (initially, paniced my iwl lab host)
Differential Revision: https://reviews.freebsd.org/D26554