Fix jtag_usb_location_equal for path members > 9

Fix jtag_usb_location_equal for path members > 9

In case an USB location path contains path members
larger than 9. The string_length variable is regardless
decremented by 2 with assumption that the member is
one digit length. For exmaple 1-12.2 will fail.
This patch uses strlen to calculate
digits in a path member.

Change-Id: I9c26a04d0c6af13fec65157f222599497294e2b2
Signed-off-by: Krzysztof Hockuba <krzysztof@hockuba.eu>
Reviewed-on: http://openocd.zylin.com/6048
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>