net: phy: micrel: lan8814: Fix when enabling/disabling 1-step timestamping

net: phy: micrel: lan8814: Fix when enabling/disabling 1-step timestamping

There are 2 issues with the blamed commit.
1. When the phy is initialized, it would enable the disabled of UDPv4
   checksums. The UDPv6 checksum is already enabled by default. So when
   1-step is configured then it would clear these flags.
2. After the 1-step is configured, then if 2-step is configured then the
   1-step would be still configured because it is not clearing the flag.
   So the sync frames will still have origin timestamps set.

Fix this by reading first the value of the register and then
just change bit 12 as this one determines if the timestamp needs to
be inserted in the frame, without changing any other bits.

Fixes: ece19502834d ("net: phy: micrel: 1588 support for LAN8814 phy")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Divya Koppera <divya.koppera@microchip.com>
Link: https://lore.kernel.org/r/20240402071634.2483524-1-horatiu.vultur@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>