stlink (tcp): manage scattered stlink-server responses

stlink (tcp): manage scattered stlink-server responses

detected in ubuntu 20.04
sometimes, the stlink-server response could be segmented on multiple
packets.

this causes stlink_tcp_send_cmd to fail with the following msg:
Error: failed to receive USB CMD response
because the received_size < expected size

to fix the issue, do recv in a loop till all data is received
or timeout is reached.

Change-Id: I46cc60c231b4cc52f150ead268f843bc60c41149
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6671
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins