be2net: Fix a bug in Rx buffer posting

The numPosted field in the ERX Doorbell register is 8-bits wide.
So the max buffers that we can post at a time is 255 and not 256
which we are doing currently.

Signed-off-by: Ajit Khaparde <ajit.khaparde@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>