octeontx2-pf: handle otx2_mbox_get_rsp errors in otx2_ethtool.c

[ Upstream commit e26f8eac ]

Add error pointer check after calling otx2_mbox_get_rsp().

Fixes: 75f36270 ("octeontx2-pf: Support to enable/disable pause frames via ethtool")
Fixes: d0cf9503 ("octeontx2-pf: ethtool fec mode support")
Signed-off-by: Dipendra Khadka <kdipendra88@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Sasha Levin <sashal@kernel.org>