tty/synclink: remove ISA support

The ISA support in this driver is horribly outdated and the last
driver that passes a NULL dev argument to the DMA mapping routines.

Drop the support, if someone wants to go through the work of converting
it to a proper isa_driver we can resurrect it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>