pwm: pca9685: Fix wrong argument to set MODE1_SLEEP bit

Current code actually does not set MODE1_SLEEP bit because the new value for
bitmask (0x1) is wrong. To set MODE1_SLEEP bit, we should pass MODE1_SLEEP
as the new value for bitmask.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>