MIPS: pic32: treat port as signed integer

get_port_from_cmdline() returns an int, yet is assigned to a char, which
is wrong in its own right, but also, with char becoming unsigned, this
poses problems, because -1 is used as an error value. Further
complicating things, fw_init_early_console() is only ever called with a
-1 argument. Fix this up by removing the unused argument from
fw_init_early_console() and treating port as a proper signed integer.

Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>