staging: r8188: make rx signal strength function static

The _linked_rx_signal_strength_display function is used only in
rtw_mlme_ext.c. Make the function static and remove the prototype.

While at it, fix a typo in the function name.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211218120423.29906-2-martin@kaiser.cx


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