(split) Always free sbuf in gctl_free().

MFC after:	1 week
(split) Get rid of memmove().  It's not portable.

Tested on RT3071, RT3573, RT3570, RT3572, and RT5572.

Reviewed by:	hselasky
(split) Fix comment.
(split) Fix a range check for maximum transmit length. The existing code was
off by 4 bytes in one case.

Approved by:	kevlo @
MFC after:	2 weeks
(split) Merge branch 'CURRENT' (commit '859fb3bb')

Conflicts:
	sys/dev/usb/wlan/if_run.c