mfd: Fix return type of lm533 attribute is_visible

Since commit 587a1f1659 ("switch ->is_visible() to returning umode_t")
the return type of is_visible is umode_t rather than mode_t.

This silences a compiler warning on some architectures where these types
are not compatible.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>