ubifs: Fix error return code in ubifs_init_authentication()

Fix to return PTR_ERR() error code from the error handling case where
ubifs_hash_get_desc() failed instead of 0 in ubifs_init_authentication(),
as done elsewhere in this function.

Fixes: 49525e5e ("ubifs: Add helper functions for authentication support")
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>