MDEV-437 Microseconds: In time functions precision is calculated modulo 256

store the precision in uint, not uint8

mysql-test/r/func_time.result

@@ -1897,3 +1897,5 @@ cast(greatest(cast("0-0-0" as date), cast("10:20:05" as time)) as datetime(6))
 select microsecond('12:00:00.123456'), microsecond('2009-12-31 23:59:59.000010');
 microsecond('12:00:00.123456')	microsecond('2009-12-31 23:59:59.000010')
 123456	10
+select now(258);
+ERROR 42000: Too big precision 258 specified for 'now'. Maximum is 6.

mysql-test/t/func_time.test

@@ -1142,3 +1142,5 @@ select cast(greatest(cast("0-0-0" as date), cast("10:20:05" as time)) as datetim
 
 select microsecond('12:00:00.123456'), microsecond('2009-12-31 23:59:59.000010');
 
+--error ER_TOO_BIG_PRECISION
+select now(258);

sql/item.h

@@ -582,8 +582,8 @@ class Item {
     calls.
   */
   uint name_length;                     /* Length of name */
+  uint decimals;
   int8 marker;
-  uint8 decimals;
   bool maybe_null;			/* If item may be null */
   bool in_rollup;                       /* If used in GROUP BY list
                                            of a query with ROLLUP */