bnxt_en: Refactor NRZ/PAM4 link speed related logic

Refactor some NRZ/PAM4 link speed related logic into helper functions.
The NRZ and PAM4 link parameters are stored in separate structure fields.
The driver logic has to check whether it is in NRZ or PAM4 mode and
then use the appropriate field.

Refactor this logic into helper functions for better readability.

Reviewed-by: Damodharam Ammepalli <damodharam.ammepalli@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>