sm501: Fix size of color_reg

color_reg is expected to hold 32 bit values, so it was too small.

This bug was reported by coverity:

hw/sm501.c:624:
result_independent_of_operands:
color_reg >> 16 is 0 regardless of the values of its operands.
This occurs as the bitwise first operand of '&'.

Cc: Shin-ichiro Kawasaki <kawasaki@juno.dti.ne.jp>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

hw/sm501.c

@@ -593,7 +593,7 @@ static inline uint32_t get_hwc_x(SM501State *state, int crt)
  */
 static inline uint16_t get_hwc_color(SM501State *state, int crt, int index)
 {
-    uint16_t color_reg = 0;
+    uint32_t color_reg = 0;
     uint16_t color_565 = 0;
 
     if (index == 0) {