ibmvnic: Clean RX pool buffers during device close

During device close or reset, there were some cases of outstanding
RX socket buffers not being freed. Include a function similar to the
one that already exists to clean TX socket buffers in this case.

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