usb: rockchip: be quiet on serial port while transferring data

While downloading or uploading megabytes of data we had thousands of
printf in console like:

transfer 0x10000 bytes done
OR
Uploading 0x1000 bytes

This because transfers are chunked and there is no way on target
side to know the overall transfer size (to print one string per
overall transfer).

All these prints on serial console do slow down significantly the
transfer and does not offer a significant information to the
developer: rkdeveloptool and Rockchip original tool do use small
chunks read/writes on big transfers. This allows on workstation
to print percentage of transfer complete and as well offers to
developer the information about: transfer is running OK.
On error, either the percentage will stop or an error will be shown
on workstation console.

Signed-off-by: Alberto Panizzo <alberto@amarulasolutions.com>