iavf: fix kernel-doc comment style in iavf_ethtool.c

iavf: fix kernel-doc comment style in iavf_ethtool.c

iavf_ethtool.c contains 31 kernel-doc comment blocks using the legacy
`**/` terminator instead of the correct single `*/`. Two function
headers also use a colon separator (`iavf_get_channels:`,
`iavf_set_channels:`) instead of the ` - ` dash required by kernel-doc.

Additionally several comments embed their return-value descriptions in
the body paragraph, producing `scripts/kernel-doc -Wreturn` warnings.
Void functions that incorrectly say "Returns ..." are also rephrased.

Fix all issues across the full file:
 - Replace every `**/` terminator with `*/`.
 - Change `function_name:` doc headers to `function_name -`.
 - Move inline "Returns ..." sentences into dedicated `Return:` sections
   for non-void functions (iavf_get_msglevel, iavf_get_rxnfc,
   iavf_set_channels, iavf_get_rxfh_key_size, iavf_get_rxfh_indir_size,
   iavf_get_rxfh, iavf_set_rxfh).
 - Rephrase body descriptions in void functions that incorrectly said
   "Returns ..." (iavf_get_drvinfo, iavf_get_ringparam, iavf_get_coalesce).
 - Remove boilerplate body text for iavf_get_rxfh_key_size and
   iavf_get_rxfh_indir_size; the `Return:` line now conveys the same
   information without the vague "Returns the table size." sentence.

Suggested-by: Anthony L. Nguyen <anthony.l.nguyen@intel.com>
Suggested-by: Leszek Pepiak <leszek.pepiak@intel.com>
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20260409093020.3808687-1-aleksandr.loktionov@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>