mips: inline serial_init()

mips: inline serial_init()

The function is specific to mipssim, let's inline it.

(when inlining, we use a DeviceState variable instead of SerialIO, as
it is the most common type used, and avoids having too many casts)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>