net: ocelot: fix wrong time_after usage

Accidentally noticed, that this driver is the only user of
while (time_after(jiffies...)).

It looks like typo, because likely this while loop will finish after 1st
iteration, because time_after() returns true when 1st argument _is after_
2nd one.

There is one possible problem with this poll loop: the scheduler could put
the thread to sleep, and it does not get woken up for
OCELOT_FDMA_CH_SAFE_TIMEOUT_US. During that time, the hardware has done
its thing, but you exit the while loop and return -ETIMEDOUT.

Fix it by using sane poll API that avoids all problems described above

Fixes: 753a026cfec1 ("net: ocelot: add FDMA support")
Suggested-by: Andrew Lunn <[e-mail]>
Signed-off-by: Pavel Skripkin <[e-mail]>
Reviewed-by: Vladimir Oltean <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/mscc/ocelot_fdma.c

···
 	ocelot_fdma_writel(ocelot, MSCC_FDMA_CH_ACTIVATE, BIT(chan));
 }
 
+static u32 ocelot_fdma_read_ch_safe(struct ocelot *ocelot)
+{
+	return ocelot_fdma_readl(ocelot, MSCC_FDMA_CH_SAFE);
+}
+
 static int ocelot_fdma_wait_chan_safe(struct ocelot *ocelot, int chan)
 {
-	unsigned long timeout;
 	u32 safe;
 
-	timeout = jiffies + usecs_to_jiffies(OCELOT_FDMA_CH_SAFE_TIMEOUT_US);
-	do {
-		safe = ocelot_fdma_readl(ocelot, MSCC_FDMA_CH_SAFE);
-		if (safe & BIT(chan))
-			return 0;
-	} while (time_after(jiffies, timeout));
-
-	return -ETIMEDOUT;
+	return readx_poll_timeout_atomic(ocelot_fdma_read_ch_safe, ocelot, safe,
+					 safe & BIT(chan), 0,
+					 OCELOT_FDMA_CH_SAFE_TIMEOUT_US);
 }
 
 static void ocelot_fdma_dcb_set_data(struct ocelot_fdma_dcb *dcb,