staging: r8188eu: Fix fall-through warnings for Clang

Fix the following fallthrough warnings:

drivers/staging/r8188eu/core/rtw_mlme_ext.c:1498:3: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
drivers/staging/r8188eu/core/rtw_wlan_util.c:1113:4: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
drivers/staging/r8188eu/core/rtw_wlan_util.c:1147:4: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
drivers/staging/r8188eu/core/rtw_wlan_util.c:1405:3: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

This helps with the ongoing efforts to globally enable
-Wimplicit-fallthrough for Clang.

Link: https://github.com/KSPP/linux/issues/115


Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210818221418.GA311735@embeddedor


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