wl18xx: add power limit reference value to mac_and_phy settings

With more recent PHY firmware versions (>8.1.0.0.116), we need to use
the correct value for the pwr_limit_reference_11_abg parameter when
setting the mac_and_phy options.

For now we use a hardcoded 0xc8 as the value.  This will be moved to
the configuration binary when it gets implemented.

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