staging: r8188eu: remove extern from function prototypes

Declaring function prototypes extern is redundant and triggers
checkpatch warnings. Remove all extern from function prototypes.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20221101191458.8619-2-straube.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>