wl12xx: make WL1271_FLAG_IDLE flag per-vif

This flag should be set per-vif, rather than globally.

Rename the flag to indicate IN_USE (rather than IDLE), as
in the default configuration (i.e. flag is clear) the vif
should be idle.

Change all the bit operations (and elp conditions) appropriately.

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