drivers: uio_dmem_genirq: Don't use DMA_ERROR_CODE to indicate unmapped regions

DMA_ERROR_CODE is not defined on all architectures and is architecture
specific.  Instead, use the constant, ~0 to indicate unmapped regions.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>
Cc: "Hans J. Koch" <hjk@hansjkoch.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>