HSI: omap_ssi_core: Fix error handling in ssi_init()

The ssi_init() returns the platform_driver_register() directly without
checking its return value, if platform_driver_register() failed, the
ssi_pdriver is not unregistered.
Fix by unregister ssi_pdriver when the last platform_driver_register()
failed.

Fixes: 0fae1989

 ("HSI: omap_ssi: built omap_ssi and omap_ssi_port into one module")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>