jtag/drivers/remote_bitbang: use sizeof to determine maximum unix socket path length

Different *nix systems use different constants, so sizeof should be
used instead. This fixes the build on OS X (as sun_path length is
hardcoded to 104 on Darwin).

Change-Id: I1df611383dedcfc6d153ec59b453a895c7d84e94
Reported-by: Xiaofan Chen <xiaofanc@gmail.com>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2095


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Xiaofan <xiaofanc@gmail.com>