bnxt_en: refactor code to limit speed advertising

Extract the code for determining an advertised speed is no longer
supported into a separate function. This will avoid some code
duplication in a later patch when supporting PAM4 speeds, since
these speeds are specified in a separate field.

Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-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>