bnxt_en: optimize gettimex64

bnxt_en: optimize gettimex64

Current implementation of gettimex64() makes at least 3 PCIe reads to
get current PHC time. It takes at least 2.2us to get this value back to
userspace. At the same time there is cached value of upper bits of PHC
available for packet timestamps already. This patch reuses cached value
to speed up reading of PHC time.

Signed-off-by: Vadim Fedorenko <vadfed@meta.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20241114114820.1411660-1-vadfed@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>