mips: serial: Fix busted manual relocation

serial_initialize() must be called after relocation to adjust the
pointers to putc(), getc(), etc.  This is busted ever since the
serial driver-model-ification series.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>