octeontx2-pf: handle otx2_mbox_get_rsp errors in otx2_dcbnl.c

[ Upstream commit 69297b0d ]

Add error pointer check after calling otx2_mbox_get_rsp().

Fixes: 8e675581 ("octeontx2-pf: PFC config support with DCBx")
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>