wcn36xx: allocate skbs with GFP_KERNEL during init

wcn36xx: allocate skbs with GFP_KERNEL during init

GFP_ATOMIC should only be used when the allocation is done from atomic
context. Introduce a new flag to wcn36xx_dxe_fill_skb() and use GFP_KERNEL
when pre-allocating buffers during init.

This doesn't fix an issue that was observed in the wild, but it reduces
the chance of failed allocations under memory pressure.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>