OMAP3: cpuidle: code rework for improved readability

- fix single and multi-lines comments format
- removed the omap3_idle_bm_check function and replaced the test
   in omap3_enter_idle_bm by the equivalent code
- re-organize omap3_enter_idle_bm code path, assign local variables
   only when needed
- reword some comments

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>