iwlwifi: pcie: merge napi_poll_msix functions

The only difference between iwl_pcie_napi_poll_msix_shared() and
iwl_pcie_napi_poll_msix() is when we have a shared queue and nothing
in the rx queue.  This case doesn't affect CPU performance, so we can
merge the two functions.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210411124417.9d1b61ef53a5.I60b33d5379cf7c12f1de30fc3fd4cefc38220141@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>