cifs: address trivial coverity warning

This is not actually a bug but as Coverity points out we shouldn't
be doing an "|=" on a value which hasn't been set (although technically
it was memset to zero so isn't a bug) and so might as well change
"|=" to "=" in this line

Detected by CoverityScan, CID#728535 ("Unitialized scalar variable")

Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>