net: dm9000: Replace DM9000_DBG() with debug()

Use standard debug() macro to print debug messages instead of
reinventing driver-specific macro again. 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>