[SERIAL] Remove struct pci_board from init_fn

Traditionally, we allocated the private array of port parameters based on
the detected board->num_ports, and then called the init_fn with the board
pointer (which points into the global table.)  Some init_fn implementations
modify num_ports, which therefore affects the global table.  Unfortunately,
this means that if the init_fn increases num_ports (because we have two
almost identical cards, the first with a smaller number of ports than the
second), we will overwrite memory which hasn't been allocated to us.

This cset no longer passes the board pointer into the init_fn, but instead
allows the init_fn to return the number of ports found, or zero to use the
default value in board->num_ports, or negative error number.