gianfar: fix compile warning

Eliminate an uninitialized variable warning.  The code is correct, but
a pointer to the automatic variable 'addr' is passed to dma_alloc_coherent.
Since addr has never been initialized, and the compiler doesn't know
what dma_alloc_coherent will do with it, it complains.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Jeff Garzik <jeff@garzik.org>