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>