power: skip echo mem if there is pending transition

power: skip echo mem if there is pending transition

For echo mem, there could be some pending transitions when it comes to
enter suspend. This is because transition is asynchronously processed.
That is, if there were another transition request during a sleep
transition, the sleep transition should be skipped and take the next
transition immediately. Otherwise, the next transition will handled
after wakeup, which is indefinite.

Change-Id: Ia09a1d1162e21710538f6e9ce7bd1fb3e9c43d0f
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>