CVE-2025-38126: net: stmmac: make sure that ptp_rate is not 0 before configuring timestamping

In the Linux kernel, the following vulnerability has been resolved:

net: stmmac: make sure that ptprate is not 0 before configuring timestamping

The stmmac platform drivers that do not open-code the clkptprate value after having retrieved the default one from the device-tree can end up with 0 in clkptprate (as clkgetrate can return 0). It will eventually propagate up to PTP initialization when bringing up the interface, leading to a divide by 0:

Division by zero in kernel. CPU: 1 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.12.30-00001-g48313bd5768a #22 Hardware name: STM32 (Device Tree Support) Call trace: unwindbacktrace from showstack+0x18/0x1c showstack from dumpstacklvl+0x6c/0x8c dumpstacklvl from Ldiv064+0x8/0x18 Ldiv064 from stmmacinittstampcounter+0x190/0x1a4 stmmacinittstampcounter from stmmachwsetup+0xc1c/0x111c stmmachwsetup from stmmacopen+0x18c/0x434 stmmacopen from stmmacopen+0x3c/0xbc stmmacopen from devopen+0xf4/0x1ac devopen from devchangeflags+0x1cc/0x224 devchangeflags from devchangeflags+0x24/0x60 devchangeflags from ipautoconfig+0x2e8/0x11a0 ipautoconfig from dooneinitcall+0x84/0x33c dooneinitcall from kernelinitfreeable+0x1b8/0x214 kernelinitfreeable from kernelinit+0x24/0x140 kernelinit from retfromfork+0x14/0x28 Exception stack(0xe0815fb0 to 0xe0815ff8)

Prevent this division by 0 by adding an explicit check and error log about the actual issue. While at it, remove the same check from stmmacptpregister, which then becomes duplicate