stlink: increase trace buffer size to maximum allowed on st-link v2 firmware

stlink: increase trace buffer size to maximum allowed on st-link v2 firmware

Increasing the trace buffer size on the st-link itself gives openocd a greater
chance of avoiding trace data overflowing within the st-link between polls
when there is a large amount of data being sent over the trace port

The st-link appears to split the given buffer size in half
while one half is awaiting transfer over USB, the other half is being
filled by DMA transfer.  If you do not poll frequently enough, the DMA
transfer will overflow back to the start of its current buffer, resulting in
corrupted output

Buffer size of 4096 bytes is the maximum allowed by the st-link v2

Change-Id: I169189b021c34f8d18de1601d78b8c5890367d68
Signed-off-by: Austin Morton <austinpmorton@gmail.com>
Reviewed-on: http://openocd.zylin.com/4085
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>