[BNX2]: Combine small mem allocations

Combine two small (56 byte and 320 byte) pci consistent memory
allocations into one allocation. Jeff Garzik suggested to store
the combined size in the bp structure for later use when freeing
the memory.

Use kzalloc() instead of kmalloc() + memset().

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>