OMAP: hwmod: Do not disable clocks if hwmod already in idle

The disable function was disabling clocks and dependencies
from both enable and idle state. Since idle function is already
disabling both, an enable -> idle -> disable sequence will
try to disable twice the clocks and thus generate a
"Trying disable clock XXX with 0 usecount" warning.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Kevin Hilman <khilman@deeprootsystems.com>