arm: fix arm reg regression

Seems commit fc2abe63 caused a regression
in that the arm reg cmd no longer worked. The issue was caused because we
changed the value of ARM_MODE_THREAD which was being checked in arm_init_arch_info.

Change-Id: Id571d4ab336d1b0e2b93363147af245d24b65ca5
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1362


Tested-by: jenkins
Reviewed-by: Luca Bruno <lucab@debian.org>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>