uio: convert to vm_fault_t

As part of commit 9b85e95a

 ("uio: Change return
type to vm_fault_t") in 4.19-rc1, this conversion
was missed. Now converted 'ret' to vm_fault_t type.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>