cmd: avb read_rb: print rb_idx in hexadecimal

Prior to this patch was do_avb_write_rb() reading supplied rb_idx as a
hexadecimal number while do_avb_read_rb() printed the read out rb_idx as
decimal number. For consistency change do_avb_read_rb() to print rb_idx
as a hexadecimal number too.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

cmd/avb.c

@@ -158,7 +158,7 @@ int do_avb_read_rb(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 
 	if (avb_ops->read_rollback_index(avb_ops, index, &rb_idx) ==
 	    AVB_IO_RESULT_OK) {
-		printf("Rollback index: %llu\n", rb_idx);
+		printf("Rollback index: %llx\n", rb_idx);
 		return CMD_RET_SUCCESS;
 	}
 	return CMD_RET_FAILURE;