stm32/mboot: Increase USB rx_buf and DFU buf sizes to full 2048 bytes.

The DFU USB config descriptor returns 0x0800=2048 for the supported
transfer size, and this applies to both TX (IN) and RX (OUT).  So increase
the rx_buf to support this size without having a buffer overflow on
received data.

With this patch mboot in USB DFU mode now works with dfu-util.