rtc-v3020: make bitfield unsigned

Fix sparse warning:

include/linux/rtc-v3020.h:18:23: error: dubious one-bit signed bitfield
Signed-off-by: NBill Pemberton <wfp5p@virginia.edu>
CC: p_gortmaker@yahoo.com
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

include/linux/rtc-v3020.h

@@ -15,7 +15,7 @@
 struct v3020_platform_data {
 	int leftshift; /* (1<<(leftshift)) & readl() */
 
-	int use_gpio:1;
+	unsigned int use_gpio:1;
 	unsigned int gpio_cs;
 	unsigned int gpio_wr;
 	unsigned int gpio_rd;