wl12xx: couple role_start_dev with roc

Device role is always started along with ROC.
Couple them together by introducing new wl12xx_start_dev
and wl12xx_stop_dev functions.

By using these functions, we solve a bug that occured during
channel switch - we started the dev role on one channel, and
ROCed on a different one.

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