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

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>