virtchnl: use u8 type for a field in the virtchnl_filter struct

The virtchnl_filter struct has a field called field_flags. A previous
commit mistakenly had the type to be a __u8. What we want is for the
field to be an unsigned 8 bit value, so let's just use the existing
kernel type u8 for that.

Signed-off-by: Harshitha Ramamurthy <harshitha.ramamurthy@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>