bus: ti-sysc: Fix handling of forced idle

For some devices we can get the following warning on boot:

ti-sysc 48485200.target-module: sysc_disable_module: invalid midlemode

Fix this by treating SYSC_IDLE_FORCE like we do for the other bits
for idlemodes mask.

Fixes: d59b6056

 ("bus: ti-sysc: Add generic enable/disable functions")
Cc: Roger Quadros <rogerq@ti.com>
Reviewed-by: Suman Anna <s-anna@ti.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>