ixgbevf: make use of BIT() macro to avoid shift of signed values

Also cleanup a case where we're bit shifting a value into place, and use
an unsigned constant. Make use of the unsigned postfix in places where
BIT() macro is not appropriate.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>