usb: class: dfu: fix USB descriptors in composite mode

In composite mode the request handler buffer is common to all functions,
and its size is defined by CONFIG_USB_COMPOSITE_BUFFER_SIZE.

Given the device is enumerated as composite in runtime mode and
as legacy in download mode, we need to define wTransferSize as
the minimum of the two.

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