urtwn(4) Random freezes, urtwn_getbuf: out of xmit buffers

[Sean Bruno]

It looks like recent HEAD seems to fail intermittently here on my home
network.  It will recover, but urtwn(4) seems to lose an ack or
something on txmit.  turning on debug, yields this message during the
hang event.

hw.usb.urtwn.debug: 1

Oct 28 12:34:58 bruno kernel: _urtwn_getbuf: _urtwn_getbuf: out of xmit
buffers
Oct 28 12:34:58 bruno kernel: urtwn_getbuf: urtwn_getbuf: stop queue
Oct 28 12:34:58 bruno kernel: _urtwn_getbuf: _urtwn_getbuf: out of xmit
buffers
Oct 28 12:34:58 bruno kernel: urtwn_getbuf: urtwn_getbuf: stop queue

Additionally, I get the following at module load time:

urtwn0: <vendor 0x0bda product 0x8176, class 0/0, rev 2.00/2.00, addr 1>
on usbus0
urtwn0: could not read efuse byte at address 0x10
urtwn0: could not read efuse byte at address 0x18