USB: Remove EXPERIMENTAL tags from some USB gadget Kconfig entries.

Based on a recent discussion on the Linux USB mailing list, remove the
designation of EXPERIMENTAL from some USB gadget entries, and tag some
of them as DEVELOPMENT.

just for fun, i added a bit of help for gadgetfs, explaining the
race condition.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>