The recent addition of timestamp correction to compensate the CDC error
introduced a subtle signed/unsigned bug in stmmac_get_tx_hwtstamp() while
it managed for some obscure reason to avoid that in stmmac_get_rx_hwtstamp().

The issue is:

    s64 adjust = 0;
    u64 ns;

    adjust += -(2 * (NSEC_PER_SEC / priv->plat->clk_ptp_rate));
    ns += adjust;

works by chance on 64bit, but falls apart on 32bit because the compiler
knows that adjust fits into 32bit and then treats the addition as a u64 +
u32 resulting in an off by ~2 seconds failure.

The RX variant uses an u64 for adjust and does the adjustment via

    ns -= adjust;

because consistency is obviously overrated.

Get rid of the pointless zero initialized adjust variable and do:

	ns -= (2 * NSEC_PER_SEC) / priv->plat->clk_ptp_rate;

which is obviously correct and spares the adjust obfuscation. Aside of that
it yields a more accurate result because the multiplication takes place
before the integer divide truncation and not afterwards.

Stick the calculation into an inline so it can't be accidentally
disimproved. Return an u32 from that inline as the result is guaranteed
to fit which lets the compiler optimize the substraction.

Cc: stable@vger.kernel.org
Fixes: 3600be5f ("net: stmmac: add timestamp correction to rid CDC sync error")
Reported-by: Benedikt Spranger <b.spranger@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Benedikt Spranger <b.spranger@linutronix.de>
Tested-by: Kurt Kanzenbach <kurt@linutronix.de> # Intel EHL
Link: https://lore.kernel.org/r/87mtm578cs.ffs@tglxSigned-off-by: Jakub Kicinski <kuba@kernel.org>