bnxt_en: reset PHC frequency in free-running mode

When using a PHC in shared between multiple hosts, the previous
frequency value may not be reset and could lead to host being unable to
compensate the offset with timecounter adjustments. To avoid such state
reset the hardware frequency of PHC to zero on init. Some refactoring is
needed to make code readable.

Fixes: 85036aee

 ("bnxt_en: Add a non-real time mode to access NIC clock")
Signed-off-by: Vadim Fedorenko <vadfed@meta.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Link: https://lore.kernel.org/r/20230310151356.678059-1-vadfed@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>