bnxt_en: Provide stored devlink "fw" version on older firmware

On older firmware that doesn't support the HWRM_NVM_GET_DEV_INFO
command that returns detailed stored firmware versions, fallback
to use the same firmware package version that is reported to ethtool.
Refactor bnxt_get_pkgver() in bnxt_ethtool.c so that devlink can call
and get the package version.

Signed-off-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>