net: stats: Fix RX time statistics update

The commit 8892406c ("kernel/sys_clock.h: Deprecate and convert
uses of old conversions") changed code to not use deprecated macro.
Unfortunately there was some changes done to RX stats update that were
missing from that commit. This commit fixes the issue and removes use
of the deprecated SYS_CLOCK_HW_CYCLES_TO_NS64() macro.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>