flash/nor/at91sam4: remove FWS=6, rename at91samg to atsamg

FWS=6 workaround removed, as this appears to be a copy-paste error
from the SAM3X family. Originally addressed in http://openocd.zylin.com/3837


but not all occurences were removed.

Atmel changed chip naming and removed 91 prefix for atsamg, samd...

Change-Id: Ia2b43da82b2ff9b1c85fdb456a0a198ab095243d
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3926


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>