TTY: amiserial, add missing platform check

When booting a multi-platform m68k kernel on a non-Amiga with
"console=ttyS0" on the kernel command line, it crashes with:

Unable to handle kernel access at virtual address 81dff01c
Oops: 00000000
PC: [<001e09a8>] serial_console_write+0xc/0x70

Add the missing platform check to amiserial_console_init() to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>