mwifiex: skip delay main work logic for USB interface.

We had introduced delay main work logic to avoid processing
interrupts when Rx pending packet count reaches high threshold.
interrupt processing is restarted later when packet count
reduces lower threashold. This helped to reduce unnecessary
overhead and improve throughput for SD and PCIe chipsets.

As there are no interrupts for USB, we will skip this logic for
USB chipsets.

Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>