armv7a_mmu: Check earlier for PAR read

armv7a_mmu: Check earlier for PAR read

Check earlier that the read of the PAR register was successful instead
of starting the decoding and then checking for an error reading that
register.

Change-Id: Id96c2b2f76d2d1c745fcfa55ad4c1e6db92106f9
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-on: http://openocd.zylin.com/5215
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>