wl18xx: increase MAX_CHANNELS_5GHZ

Some regdomains have more than 23 valid 5ghz channels,
so 18xx's MAX_CHANNELS_5GHZ was increased to 32.

Since now we have different max 5ghz channels values
for wl12xx and wl18xx, add a new wl->max_channels_5ghz
field, and use it for scan channels configuration.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>