iavf: refactor add/del FDIR filters

In preparation for a second type of FDIR filters that can be added by
tc-u32, move the add/del of the FDIR logic to be entirely contained in
iavf_fdir.c.

The iavf_find_fdir_fltr_by_loc() is renamed to iavf_find_fdir_fltr()
to be more agnostic to the filter ID parameter (for now @loc, which is
relevant only to current FDIR filters added via ethtool).

The FDIR filter deletion is moved from iavf_del_fdir_ethtool() in
ethtool.c to iavf_fdir_del_fltr(). While at it, fix a minor bug where
the "fltr" is accessed out of the fdir_fltr_lock spinlock protection.

Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Reviewed-by: Marcin Szycik <marcin.szycik@linux.intel.com>
Signed-off-by: Ahmed Zaki <ahmed.zaki@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>