Amiga serial driver: port_write_mutex fixup

The port_write_mutex was converted from a semaphore to a mutex,
but there was still this ifdef'd init_MUTEX reference remaining.

Signed-off-by: Daniel Walker <dwalker@mvista.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>