Staging: Gasket: uninitialized return in gasket_mmap()

We forgot to set the error code on this error path so ret can be
uninitialized.

Fixes: 9a69f508 ("drivers/staging: Gasket driver framework + Apex driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>