octeontx2-pf: handle otx2_mbox_get_rsp errors in otx2_common.c

[ Upstream commit 0fbc7a50 ]

Add error pointer check after calling otx2_mbox_get_rsp().

Fixes: ab58a416 ("octeontx2-pf: cn10k: Get max mtu supported from admin function")
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>