net/ixgbevf: set the interrupt interval for EITR

net/ixgbevf: set the interrupt interval for EITR

Set EITR interval as default. This patch can improve the
performance when we enable the rx-interrupt to process the
packets because we hope rx-interrupt reduce CPU. For example,
the 200us value of EITR makes the performance better with
the low CPU. The default value of ITR is 500us, compatible
with RSC of ixgbe PF, and next patch will use the default value.

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>