serial_core: Fix type definition for PORT_BRCM_TRUMANAGE.

It was mistakenly defined to be 24 instead of the next higher number 25.

Reported-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Stephen Hurd <shurd@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Cc: stable <stable@vger.kernel.org>  # 3.8
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>