HID: fix sparse warnings

Fix these sparse warnings:
.../hid/hid-core.c:100:15: warning: incorrect type in assignment (different signedness)
.../hid/hid-core.c:100:15:    expected signed int [usertype] *value
.../hid/hid-core.c:100:15:    got unsigned int *<noident>
        by unsigned -> s32

.../hid/hid-input-quirks.c:336:10: warning: Using plain integer as NULL pointer
        by 0 -> NULL

.../hid/usbhid/hid-core.c:786:46: warning: incorrect type in argument 3 (different signedness)
.../hid/usbhid/hid-core.c:786:46:    expected int *max
.../hid/usbhid/hid-core.c:786:46:    got unsigned int *<noident>
.../hid/usbhid/hid-core.c:787:47: warning: incorrect type in argument 3 (different signedness)
.../hid/usbhid/hid-core.c:787:47:    expected int *max
.../hid/usbhid/hid-core.c:787:47:    got unsigned int *<noident>
.../hid/usbhid/hid-core.c:788:48: warning: incorrect type in argument 3 (different signedness)
.../hid/usbhid/hid-core.c:788:48:    expected int *max
.../hid/usbhid/hid-core.c:788:48:    got unsigned int *<noident>
        by int -> unsigned int

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index 538bf28..d7b2f68 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -97,7 +97,7 @@
 	field->index = report->maxfield++;
 	report->field[field->index] = field;
 	field->usage = (struct hid_usage *)(field + 1);
-	field->value = (unsigned *)(field->usage + usages);
+	field->value = (s32 *)(field->usage + usages);
 	field->report = report;
 
 	return field;