ibmvnic: Allocate number of rx/tx buffers agreed on by firmware

The amount of TX/RX buffers that the vNIC driver currently allocates
is different from the amount agreed upon in negotiation with firmware.
Correct that by allocating the requested number of buffers confirmed
by firmware.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>