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 and leave us with one unlock/lock cycle as well.
PR: 249925
Reported by: thj, (rkoberman gmail.com)
Suggested by: adrian
Reviewed by:
Sponsored by: The FreeBSD Foundation (initially)
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