ioatdma: silence GCC warnings

Building dma_v3.o triggers a GCC warning:
    drivers/dma/ioat/dma_v3.c: In function ‘__ioat3_prep_pq16_lock’:
    drivers/dma/ioat/dma_v3.c:264:11: warning: array subscript is below array bounds [-Warray-bounds]
    drivers/dma/ioat/dma_v3.c:264:11: warning: array subscript is below array bounds [-Warray-bounds]

This warning is caused by pq16_set_src(). It uses "int idx" as an index
to an eight element array. Changing "idx" to "unsigned" silences this
warning. Apparently GCC can then determine that "idx" will never be
negative.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <djbw@fb.com>