wl12xx: use wiphy values for setting rts, frag thresholds on init

Use the wiphy RTS and fragmentation thresholds for initializing the FW
when possible. This mitigates a bug where previously set values are
forgotten after interface down/up.

Add checks before settings these values to ensure they are valid. Use
default values when invalid thresholds are configured.

Update the default RTS threshold to the maximum value given by the
specification.

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