iwlwifi: mvm: use a cast to calculate the last seqno from the next one

If the next seqno returned by the firmware is 0, we return an error
(-16) in the iwl_mvm_get_last_nonqos_seq() function.  This is because
we return an integer and don't use any casting when calculating the
last seqno from the one we received.  Fix this by using a cast to u16
when doing the calculation, so we return 0xfff0, as we should.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>