8250_pci: Remove duplicate struct pciserial_board

pbn_exsys_4055 is the same thing as pbn_b2_4_115200 so replace it with
the standard pattern.

Signed-off-by: Shawn Bohrer <shawn.bohrer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>