vfio/fsl-mc: fix the return of the uninitialized variable ret

The vfio_fsl_mc_reflck_attach function may return, on success path,
an uninitialized variable. Fix the problem by initializing the return
variable to 0.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: f2ba7e8c ("vfio/fsl-mc: Added lock support in preparation for interrupt handling")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Diana Craciun <diana.craciun@oss.nxp.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>