Remove AppleTalk support.

AppleTalk was a network transport protocol for Apple Macintosh devices
in 80s and then 90s. Starting with Mac OS X in 2000 the AppleTalk was
a legacy protocol and primary networking protocol is TCP/IP. The last
Mac OS X release to support AppleTalk happened in 2009. The same year
routing equipment vendors (namely Cisco) end their support.

Thus, AppleTalk won't be supported in FreeBSD 11.0-RELEASE.
Similar to r246614, fix panic on removing urtwn(4).  It happens because
nodes are freed after the adapter is gone.
Reset the bit of the R92C_MCUFWDL associated with checksum report
before loading firmware page.  It may fix this problem:
"urtwn0: timeout waiting for checksum report"