bnxt_en: Return -EOPNOTSUPP for ETHTOOL_GREGS on VFs.

Debug firmware commands are not supported on VFs to read registers.
This patch avoids logging unnecessary access_denied error on VFs
when user calls ETHTOOL_GREGS.

By returning error in get_regs_len() method on the VF, the get_regs()
method will not be called.

Fixes: b5d600b0 ("bnxt_en: Add support for 'ethtool -d'")
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>