wl18xx: clean up phy module parameters

Give all wl18xx phy module paramters -1 as a default value, indicating
the paramter was not set. Add previous default values to the default
18xx priv conf structure.

Remove the board_type field from wl18xx priv. The field with the same
name inside the phy conf is good enough for our purposes.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>