mwifiex: fix out of memory issue observed for USB chipsets

On some platforms, system goes out of memory during heavy
Rx traffic with our USB chipsets.

In case of SDIO/PCIe, after receiving 50 packets in Rx queue
we stop processing interrupts till packets pending fall below
low threshold i.e 20. We don't have similar logic for USB,
so if host platform is slow, we would hit a case where firmware
keeps on pushing packets at high speed than driver/kernel can
process.

We will stop submitting URBs for Rx data when pending packet
count reaches high threshold and restart them when enough
packets are consumed to solve the problem.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=85071


Reported-by: Marek Belisko <marek.belisko@gmail.com>
Tested-by: Marek Belisko <marek.belisko@gmail.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>