ath10k: stop abusing GFP_DMA

Allocations from the DMA zone were originally added for legacy ISA
stuff, or PCI devices that have specific limitations in their DMA
addressing capabilities. It has no place in ath10k, which can do
full 32-bit DMA.

Fixes memory allocation errors on some platforms.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>