usb: gadget: move choice ... endchoice to legacy/Kconfig

drivers/usb/gadget/Kconfig includes drivers/usb/gadget/legacy/Kconfig
inside the 'choice' block. The current Kconfig allows this, but I'd
like to discourage this usage.

People tend to mess up the structure without noticing that entire
drivers/usb/gadget/legacy/Kconfig is placed in the choice context.
In fact, legacy/Kconfig mixes up bool and tristate in the choice,
and creates nested choice, etc.

This commit does not change the behavior, but it will help people
notice how badly this Kconfig file is written.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20191211073857.16780-1-masahiroy@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>