mtd: phram: only call platform_driver_unregister if phram_setup fails

The check on ret and call to platform_driver_unregister is only required
in the code path where MODULE is not defined. Fix this by moving this
code into the relevant place after the call to phram_setup.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>