misc: ibmasm: fix build errors

Fix build when CONFIG_SERIAL_8250=m and CONFIG_IBM_ASM=y.

Fixes these build errors:

  drivers/built-in.o: In function `ibmasm_remove_one':
  module.c:(.text+0xf6874): undefined reference to `ibmasm_unregister_uart'
  drivers/built-in.o: In function `ibmasm_init_one':
  module.c:(.text+0xf6c37): undefined reference to `ibmasm_register_uart'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Max Asbock <masbock@us.ibm.com>
Cc: Vernon Mauery <vernux@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>