net: dm9000: Turn DM9000_DMP_PACKET() into a function

Rework DM9000_DMP_PACKET() into dm9000_dump_packet() function,
this brings better type checking. 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>