net: mscc: ocelot: divide watermark value by 60 when writing to SYS_ATOP

Tail dropping is enabled for a port when:

1. A source port consumes more packet buffers than the watermark encoded
   in SYS:PORT:ATOP_CFG.ATOP.

AND

2. Total memory use exceeds the consumption watermark encoded in
   SYS:PAUSE_CFG:ATOP_TOT_CFG.

The unit of these watermarks is a 60 byte memory cell. That unit is
programmed properly into ATOP_TOT_CFG, but not into ATOP. Actually when
written into ATOP, it would get truncated and wrap around.

Fixes: a556c76a ("net: mscc: Add initial Ocelot switch support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>