cortex_m: Check return value from cortex_m_dcc_read.

Caught by clang.

Change-Id: I26d2b845aca431612862ef432b217ca397d9b893
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2279
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>