iwlwifi: mvm: fix TSO with highly fragmented SKBs

Our hardware has a limited amount of buffer descriptors
for each Tx packet. Because of that, there is a short
piece of code that makes sure that that we don't push too
many subframes in an A-MSDU because of subframes needs 2
buffer descriptors. This code also takes into account the
number of fragment of the skb since we also need a buffer
descriptor for each fragment in the skb.

This piece of code though didn't check that the resulting
number of subframes wasn't 0.

A user reported that using NFS client, he could get skbs
that are so fragmented that the code mentioned above
returned 0 for the number of subframes making
skb_gso_segment fail and subconsequently iwlwifi would WARN.

Fix this by make sure that num_subframes is at least 1.

This fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=199209



Fixes: a6d5e32f ("iwlwifi: mvm: send large SKBs to the transport")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>