x86: only compile 8237A if CONFIG_ISA_DMA_API is enabled

8237A utilizes the interface provided by CONFIG_ISA_DMA_API, specifically
claim_dma_lock() and release_dma_lock().  Thus, there's a strict
dependency on the config option and the module should only be loaded if
the kernel supports ISA-style DMA.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>