bnxt_en: Refactor ethtool speeds logic

Add helper functions to refactor the logic that converts firmware
speed masks to ethtool speeds.  Pass the phy_flags to
bnxt_get_ethtool_speeds() and the call chain.  The refactoring and the
phy_flags will be needed when adding support for the new speeds in the
next patches.

Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20231201223924.26955-12-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>