flash/spi_flash_w25qxxdv: do not set driver_api if init fails

If the driver fails initializations, do not set the driver_api.
This follows the idea that device_get_binding() will not return
a reference to the driver instance when driver init fails.

Change-Id: Iff56b4150658a76567928b1fd166bde5d2848d52
Signed-off-by: Daniel Leung <daniel.leung@intel.com>