liquidio: fix hang when re-binding VF host drv after running DPDK VF driver

When configuring SLI_PKTn_OUTPUT_CONTROL, VF driver was assuming that IPTR
mode was disabled by reset, which was not true.  Since DPDK driver had
set IPTR mode previously, the VF driver (which uses buf-ptr-only mode) was
not properly handling DROQ packets (i.e. it saw zero-length packets).

This represented an invalid hardware configuration which the driver could
not handle.

Signed-off-by: Rick Farrington <ricardo.farrington@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>