cfi: fix type-punning warnings in cfi_spansion_write_block

cfi: fix type-punning warnings in cfi_spansion_write_block

Retest the condition when needed, instead of abusing the common_magic
field as a flag. There are only two options here. Either it's an armv7m or
it's another arm. is_arm(...) will return true even for armv7m, so it's
imperative to check in the right order.

Change-Id: Ic227f19f7babf1b0b0fe075f9a3abc4eabc7d5f1
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/779
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>