imx_sdp: implement expected receive size

Distinguish between receive buffer size and expected message size
according to SDP specification. In USB mode, the device seems to
send more data than specified: Using a buffer of size 4 for report
4 leads to error LIBUSB_ERROR_OVERFLOW (-8). However in UART mode
only 4 bytes are received.

Having both information, buffer size and expected data size, allows
us to read only specified size in UART mode while using the
whole buffer size in USB mode.
imx_sdp: less debug print

Remove some debug print from do_status, but print result of do_status
and file download.