stlink: remove usb timeout magic numbers

Change-Id: I4ec4cb0bc344c4c1de9113fa12ed84e5e2e87b31
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1631


Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>