Fix: Error while reading from USB endpoint

Fix: Error while reading from USB endpoint

This patch fixes the bug reported here:

http://sourceforge.net/mailarchive/message.php?msg_id=28350157

When using Rlink under Linux, openocd exits with:

"Error: Read of endpoint 2 returned -75, expected 17"

The return value of -75 translates into EOVERFLOW. The cause is a wrong output buffer size argument passed to dtc_run_download().

Change-Id: I5d056705181ab6a6d4355524df06a0ea9c605961
Signed-off-by: Peter Horn <peter.horn@bluewin.ch>
Reviewed-on: http://openocd.zylin.com/862
Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>