I2C: OMAP: remove dev->idle, use usage counting provided by runtime PM

Current usage of runtime PM is not quite correct.  The actual
idle/unidle of the I2C hardware should not happen until the runtime PM
callbacks are called.  Therefore, change omap_i2c_[un]idle() functions
to only be called from the runtime PM callbacks (when usage count
transitions to/from zero.)

Also, the runtime PM core does usage counting and replaces
functionality currently managed by the dev->idle flag.  Remove usage
of dev->idle in favor of using runtime PM, and checking status using
pm_runtime_suspended().

Signed-off-by: Kevin Hilman <khilman@ti.com>