gdb server: Fix bug. Parse 'M' packet error.

The format of 'M' packet is 'M addr,length:XX...'. The data
follows ':' immediately. No need to '+2' to SEPARATOR in
unhexify(), because SEPARATOR points to data correctly.

Change-Id: I15b5758b540816cc727752e7bf68cd45e623f603
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1360
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>