pwm/pca_9685: move driver_api assignment later

This moves the assignment of driver_api to just before
the init function returns. This is in preparation to
make device_get_binding() return NULL if driver
initialization fails.

Change-Id: Ia0c7a0014eb28624cbf363f994d6149f1aa5aadd
Signed-off-by: Daniel Leung <daniel.leung@intel.com>