staging: r8188eu: fix the declaration of process_pwrbit_data

Fix the declaration of process_pwrbit_data. Make the function static and
remove its prototype.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Pavel Skripkin <paskripkin@gmail.com>
Link: https://lore.kernel.org/r/20220602194807.281115-4-martin@kaiser.cx


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