net: dm9000: Drop static device private data

Allocate driver private data dynamically in its init function and drop
the static driver private data variable. Pass the dynamic private data
throughout the driver. This is done in preparation for DM conversion.

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>