serial: 8250_pci: all known Braswell ports are 1 channel

There is no need to have channel offset defined since all BayTrail and Braswell
ports are 1 channel. Remove unneeded definition.

While here, remove comment which has no value.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>