staging: unisys: visorchannel.c remove unneeded parenthesis

Fix the checkpatch.pl -strict check:

CHECK: Unnecessary parentheses around sig_hdr.num_overflows
+                                  &(sig_hdr.num_overflows),

Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>