net: dm9000: Receive one packet per recv call

Instead of reading out the entire FIFO and possibly overwriting U-Boot
memory, read out one packet per recv call, pass it to U-Boot network
stack, and repeat.

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>