ice: keep the RSS hash function on indirection-only updates

The ethtool core passes RXH_XFRM_NO_CHANGE (0xff) in input_xfrm when a
request does not touch the input transformation, e.g. when it only
rewrites the RSS indirection table. ice_set_rxfh() tests input_xfrm only
for RXH_XFRM_SYM_XOR, which 0xff has set, so such an update switches the
VSI to symmetric Toeplitz hashing behind the user's back.

Once symmetric-xor is reported, the core requires all hashed flow types
to be symmetric for any following RSS set. The GTP flow types hash on
the TEID, so every later netlink RSS update fails with "hash field config
is not symmetric", including the one that would put the indirection
table back.

Keep the current hash function when input_xfrm is RXH_XFRM_NO_CHANGE.
ice_set_rss_hfunc() then returns early and leaves the VSI as it is.

This has been encountered while trying to run xskxceiver within drv-net
tests where one of the setup steps was queue pull out of RSS indirection
table.

Fixes: 352e9bf23813 ("ice: enable symmetric-xor RSS for Toeplitz hash function")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>