iwlwifi: do not use GFP_DMA in iwl_tx_queue_init

GFP_DMA is not necessary for the iwlwifi hardware and it can cause
allocation failures and/or invoke the OOM killer on lots of systems.

For reference:

	https://bugzilla.redhat.com/show_bug.cgi?id=459709

Signed-off-by: John W. Linville <linville@tuxdriver.com>