usb: use "prompt" instead of "bool" for choice prompts

usb: use "prompt" instead of "bool" for choice prompts

Since commit fde192511bdb ("kconfig: remove tristate choice support"),
all choice blocks are now boolean. There is no longer a need to specify
the choice type explicitly.

Most choice blocks already use "prompt". Before the next commit removes
support for the "bool" syntax in choice entries, this commit converts
the remaining "bool" occurences under drivers/usb/.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>