OMAP24xx: CM: fix mask used for checking IDLEST status

On OMAP24xx, the polarity for the IDLEST bits is opposite of OMAP3.
The mask used to check this was using the bit position instead of the
bit mask.

This patch fixes the problem by using the bit mask instead of the bit
field.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>