net: dm9000: Rename board_info to dm9000_priv

Rename board_info structure to dm9000_priv to make it clear what this
structure really contains, the driver private data. 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>