mlxsw: spectrum: Do not assume PAUSE frames are disabled

When ieee_setpfc() gets called, PAUSE frames are not necessarily
disabled on the port.

Check if PAUSE frames are disabled or enabled and configure the port's
headroom buffer accordingly.

Fixes: d81a6bdb

 ("mlxsw: spectrum: Add IEEE 802.1Qbb PFC support")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>