bnxt_en: Return proper error code for non-existent NVM variable

[ Upstream commit 05069dd4c577f9b143dfd243d55834333c4470c5 ]

For NVM params that are not supported in the current NVM
configuration, return the error as -EOPNOTSUPP.

Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>