sfc: unfix not-a-typo in comment

sfc: unfix not-a-typo in comment

Commit fe09560f8241 ("net: Fix typos") removed duplicated word 'fallback',
 but this was not a typo and change altered the semantic meaning of
 the comment.
Partially revert, using the phrase 'fallback of the fallback' to make
 the meaning more clear to future readers so that they won't try to
 change it again.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250731144138.2637949-1-edward.cree@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>