ipa: fix ipa3_send_nop_desc locking before alloc

After adding the tx_pkt_wrapper cache the allocation
was done in GFP_KERNEL while SoftIrqs are disabled
since spin_lock_bh was used.
This change releases the spinlock before allocation is done.

Change-Id: I3c3cee3e82d581c79a0258d3fcec378ed3d9a07c
Acked-by: Tal Gelbard <tgelbard@qti.qualcomm.com>
Signed-off-by: Amir Levy <alevy@codeaurora.org>