Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

urtw: switch to ieee80211_add_channel_list_2ghz().

- Use device's channel list instead of default one (from
ieee80211_init_channels()).
- Add ic_getradiocaps() method.

Fix undefined behavior: (1 << 31) is not defined as 1 is an int and this
shifts into the sign bit. Instead use (1U << 31) which gets the
expected result.

This fix is not ideal as it assumes a 32 bit int, but does fix the issue
for most cases.

A similar change was made in OpenBSD.

Discussed with: -arch, rdivacky
Reviewed by: cperciva

overhauls urtw(4) for supporting RTL8187B devices properly that there
was major changes to initialize RF chipset and set H/W registers and
removed a lot of magic numbers on code. Details are as follows:

- uses the endpoint 0x89 to get TX status information which used to
get TX complete or retry numbers or get a beacon interrupt. It's
only valuable for RTL8187B.
- removes urtw_write[8|16|32]_i functions that it's useless now.
- uses ic->ic_updateslot to set SLOT, SIFS, DIES, EIFS, CW_VAL
registers that doesn't set these whenever the channel is changed.
- code for initializing RF chipset for RTL8187B changed a lot that
there was many problems on TX transfers so it doesn't work properly
even if just for a ping/pong. Now it becomes more stable than
before that TX throughputs using netperf(1) were about 15 ~ 17Mbps/s
though sometimes it encounters packet losses.
- removes a lot of magic numbers that in the previous all of
representing RX and TX descriptors were consisted of magic numbers
and structures. It'd be more readable rather than before.
- calculates TX duration more accurately for urtw(4) devices.
- style(9)

ports urtw(4) for USB2. Additionally it supports a 8187B chipset weakly
that it needs more stabilization.