usbconfig: documentation fixes, mainly for -i option

• in usage(), clearly mark -i interface as optional
• both, -u busnum and -a devaddr are optional as well
• various minor man page fixes
• clearly mark those two commands that actually use -i ifaceidx
• remove unused bitfield tag got_iface
• fix indentation level according to review comment

Differential Revision: https://reviews.freebsd.org/D33579/
Reviewed by: hselasky
MFC after: 2 weeks