rlink: Fix DTC command timeout

rlink: Fix DTC command timeout

With the current timeout setting i = 10 in drtc_run_timeout()
I get "Error: too many retries waiting for DTC status" when
loading a program into the FLASH of an STM32F1.

By experimentation a value of i = 22 was found to be the minimum
on my system. Therefore the value has been increased to i = 50.

Change-Id: Ib67fc648ccaad305871b81c2c39e49de53c330a0
Signed-off-by: Peter Horn <peter.horn@bluewin.ch>
Reviewed-on: http://openocd.zylin.com/863
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>