[RADEON]: Fix unaligned I/O port access during probe.

The driver does a readl() on DEVICE_ID which is 2-byte aligned and
2-bytes in size.  It's doing this read just to flush write buffers.

Create IN16() and OUT16() macros, and use the former to do this I/O
load.

Signed-off-by: David S. Miller <davem@davemloft.net>