bnxt_en: PTP: Refactor PTP initialization functions

Making the ptp free and timecounter initialization code into separate
functions so that later patches can use them.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>