net: dm9000: Correctly handle empty FIFO

Assign packet pointer only in case the MAC reports anything in the FIFO.
In case the MAC indicates empty FIFO, return 0 to pass that information
to the network stack.

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