usb: move the request handler buffer to the USB device code

In order to unify the legacy and composite code, move the class and
vendor request handler buffer into the USB device code, just like in
composite mode. The option is renamed from USB_COMPOSITE_BUFFER_SIZE
into USB_REQUEST_BUFFER_SIZE and also replaces the USB_DFU_MAX_XFER_SIZE
and USB_HID_MAX_PAYLOAD_SIZE options.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>