net: wwan: iosm: fix crash in peek throughput test

Peek throughput UL test is resulting in crash. If the UL
transfer block free list is exhaust, the peeked skb is freed.
In the next transfer freed skb is referred from UL list which
results in crash.

Don't free the skb if UL transfer blocks are unavailable. The
pending skb will be picked for transfer on UL transfer block
available.

Fixes: 1f52d7b6 ("net: wwan: iosm: Enable M.2 7360 WWAN card support")
Signed-off-by: M Chetan Kumar <m.chetan.kumar@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>