rtc-v3020: make bitfield unsigned

Fix sparse warning:

include/linux/rtc-v3020.h:18:23: error: dubious one-bit signed bitfield

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
CC: p_gortmaker@yahoo.com
Signed-off-by: Jiri Kosina <jkosina@suse.cz>