iwlwifi: rs-4965 fix return values

This patch cleans up style of 'return' of rate scale 4965 functions.
It renames return name variables rc to ret, change functions to void if no
meaningful value was returned it add return of -EINVAL for checks of wrong
arguments.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>