ixgbevf: minor cleanups for ixgbevf_set_itr()

adapter->rx_itr_setting is not a mask so check it with == instead of &
do not default to 12K interrupts in ixgbevf_set_itr()

There should be no functional effect from these changes.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>