ubi: Fix error for write access

When opening a device with write access, ubiblock_open returns an error
code. Currently, this error code is -EPERM, but this is not the right
value.

The open function for other block devices returns -EROFS when opening
read-only devices with FMODE_WRITE set. When used with dm-verity, the
veritysetup userspace tool is expecting EROFS, and refuses to use the
ubiblock device.

Use -EROFS for ubiblock as well. As a result, veritysetup accepts the
ubiblock device as valid.

Cc: stable@vger.kernel.org
Fixes: 9d54c8a3

 (UBI: R/O block driver on top of UBI volumes)
Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>