usb_blaster: fix warning: array subscript has type ‘char’

caught by clang 3.4 running on cygwin.

Change-Id: I59f652337334e557fb50374f5270ba9c30392b6e
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2100


Reviewed-by: Jörg Wunsch <openocd@uriah.heep.sax.de>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins