net: mscc: ocelot: convert SYS_PAUSE_CFG register access to regfield

Seville has a different bitwise layout than Ocelot and Felix.

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>