[POWERPC] More DCR native fixups

Getting BenH's new EMAC driver working on 440GP, I found some more
problems in the native mode paths of the new DCR code:
	- dcr_map() is supposed to return a dcr_host_t, but the native
version is a macro that doesn't expand to an expression.  With native
DCRs, dcr_host_t is an empty structure, so we just use a constructor
expression instead.
	- dcr_unmap() uses {} instead of the safer do {} while (0)
idiom to implement a no-op

Here's a fix.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>