USB: ftdi_sio: fix minor typo in get_ftdi_divisor

Even if it's unlikely for this to cause an error,
there is a typo in the code that uses the bitwise-AND
operator instead of the logical one.

Signed-off-by: Ionut Nicu <ionut.nicu@cloudbit.ro>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>