generic: fix kernel warning no previous prototype for ...

generic: fix kernel warning no previous prototype for ...

It seems new kernel version introduced -Wmissing-prototypes. This new
warning reported drivers that define non static function that are used
statically in the driver.

Fix this by declaring making those function actually static if not
defined in any header and not used outside of the single driver.

Co-authored-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Link: https://github.com/openwrt/openwrt/pull/18455
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>