usb: merge all request handler buffers

Now that we have merged the class and vendor request buffers into the
USB device code, we can also merge the standard request buffer. We just
need to ensure it is at least 8 bytes, by using the range option in
Kconfig. The 64KiB limit is arbitrary.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>