net: dm9000: Drop volatiles

Remove volatile keyword usage from arrays, they are not really volatile
in any way, so this keyword is misused here. No functional change.

Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Ramon Fried <rfried.dev@gmail.com>