wlcore: restore default channel configuration

wlcore allocates two static structs wl1271_band_2ghz & wl1271_band_5ghz
which are used/modified by Reg-Domain e.g. some channel might be marked
as passive at some point. Make sure we don't keep stale settings around
if the HW is unregistered/registered during operation.

[Arik - use Tx-power constant and tweak commit message]

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