rtwn: add workaround sleep in r92e_set_chan()

It /looks/ like there's some weirdness in initial frame send after
the chip programming / channel change. Linux rtl8xxxu has no sleeps
here, instead it just retries the auth frame a few times.

My guess is there's some sequencing going on here between finishing
the programming, doing a calibration run and then sending the initial
frame.

Instead of doing sleeps on every write during the RF channel change,
this 10ms sleep at the end is enough to reliably associate in my
test environment (12-core intel laptop, USB-3 port.) It's not
required for an earlier 2-core haswell laptop w/ USB-3.

See the PR for more information.

PR: kern/247528
Differential Revision: https://reviews.freebsd.org/D48517