iwlwifi: stop passing bogus gfp flags arguments to dma_alloc_coherent

dma_alloc_coherent is not just the page allocator.  The only valid
arguments to pass are either GFP_ATOMIC or GFP_ATOMIC with possible
modifiers of __GFP_NORETRY or __GFP_NOWARN.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>