octeontx2-pf: Change receive buffer size using ethtool

ethtool rx-buf-len is for setting receive buffer size,
support setting it via ethtool -G parameter and getting
it via ethtool -g parameter.

Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>