octeontx2-pf: handle otx2_mbox_get_rsp errors in otx2_flows.c

Adding error pointer check after calling otx2_mbox_get_rsp().

Fixes: 9917060f ("octeontx2-pf: Cleanup flow rule management")
Fixes: f0a1913f ("octeontx2-pf: Add support for ethtool ntuple filters")
Fixes: 674b3e16 ("octeontx2-pf: Add additional checks while configuring ucast/bcast/mcast rules")
Signed-off-by: Dipendra Khadka <kdipendra88@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>