net: macb: Optimize reading HW timestamp

The seconds input from BD (6 bits) just needs to be ORed with the
upper bits from timer in this function. Avoid addition operation
every single time. Seconds rollover handling is left untouched.

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>