serial: vt8500_serial: Fix a parameter of find_first_zero_bit.

The 2nd parameter of 'find_first_zero_bit' is the number of bits to search.
In this case, we are passing 'sizeof(vt8500_ports_in_use)'.
'vt8500_ports_in_use' is an 'unsigned long'. So the sizeof is likely to
return 4 on a 32 bits kernel.

A few lines below, we check if it is below VT8500_MAX_PORTS, which is 6.

It is likely that the number of bits in a long was expected here.

In order to fix it:
   - use DECLARE_BITMAP when declaring the vt8500_ports_in_use
   - use VT8500_MAX_PORTS as a maximum value when checking/setting bits in
     this bitmap
   - modify code now that 'vt8500_ports_in_use' has become a pointer
     because of the use of DECLARE_BITMAP

It has been spotted by the following coccinelle script:
@@
expression ret, x;

@@
*  ret = \(find_first_bit \| find_first_zero_bit\) (x, sizeof(...));
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/vt8500_serial.c

@@ -118,7 +118,7 @@ struct vt8500_port {
  * have been allocated as we can't use pdev->id in
  * devicetree
  */
-static unsigned long vt8500_ports_in_use;
+static DECLARE_BITMAP(vt8500_ports_in_use, VT8500_MAX_PORTS);
 
 static inline void vt8500_write(struct uart_port *port, unsigned int val,
 			     unsigned int off)
@@ -663,15 +663,15 @@ static int vt8500_serial_probe(struct platform_device *pdev)
 
 	if (port < 0) {
 		/* calculate the port id */
-		port = find_first_zero_bit(&vt8500_ports_in_use,
-					sizeof(vt8500_ports_in_use));
+		port = find_first_zero_bit(vt8500_ports_in_use,
+					   VT8500_MAX_PORTS);
 	}
 
 	if (port >= VT8500_MAX_PORTS)
 		return -ENODEV;
 
 	/* reserve the port id */
-	if (test_and_set_bit(port, &vt8500_ports_in_use)) {
+	if (test_and_set_bit(port, vt8500_ports_in_use)) {
 		/* port already in use - shouldn't really happen */
 		return -EBUSY;
 	}