wl12xx: make WL1271_FLAG_PSM flag per-vif

move WL1271_FLAG_PSM and WL1271_FLAG_PSM_REQUESTED into
per-vif flags.
These flags should be set per-vif, rather than globally.

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