staging: r8188eu: make c2h_evt_read() static

staging: r8188eu: make c2h_evt_read() static

The function c2h_evt_read() is only used in rtw_cmd.c.
Make it static.

This addresses the TODO item:
* Remove the HAL layer and migrate its functionality into the relevant
  parts of the driver.

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/20220909100232.8305-1-straube.linux@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>