net: rtl8192cu: Fix implicit fallthrough warnings

These warnings appear with GCC 7.3.0 from toolchains.bootlin.com:

../drivers/net/wireless/realtek/rtl8192cu/core/rtw_mlme_ext.c: In function ‘mgt_dispatcher’:
../drivers/net/wireless/realtek/rtl8192cu/core/rtw_mlme_ext.c:734:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if(check_fwstate(pmlmepriv, WIFI_AP_STATE) == _TRUE)
      ^
../drivers/net/wireless/realtek/rtl8192cu/core/rtw_mlme_ext.c:739:3: note: here
   case WIFI_ASSOCREQ:
   ^~~~
../drivers/net/wireless/realtek/rtl8192cu/hal/rtl8192c/rtl8192c_phycfg.c: In function ‘phy_TxPwrIdxToDbm’:
../drivers/net/wireless/realtek/rtl8192cu/hal/rtl8192c/rtl8192c_phycfg.c:2365:10: warning: this statement may fall through [-Wimplicit-fallthrough=]
   Offset = -8;
   ~~~~~~~^~~~
../drivers/net/wireless/realtek/rtl8192cu/hal/rtl8192c/rtl8192c_phycfg.c:2366:2: note: here
  default:
  ^~~~~~~
../drivers/net/wireless/realtek/rtl8192cu/hal/rtl8192c/usb/usb_halinit.c: In function ‘GetHwReg8192CU’:
../drivers/net/wireless/realtek/rtl8192cu/hal/rtl8192c/usb/usb_halinit.c:5694:20: warning: this statement may fall through [-Wimplicit-fallthrough=]
    *((u16 *)(val)) = pHalData->BasicRateSet;
    ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/wireless/realtek/rtl8192cu/hal/rtl8192c/usb/usb_halinit.c:5695:3: note: here
   case HW_VAR_TXPAUSE:
   ^~~~
../drivers/net/wireless/realtek/rtl8192cu/os_dep/linux/ioctl_linux.c: In function ‘set_group_key’:
../drivers/net/wireless/realtek/rtl8192cu/os_dep/linux/ioctl_linux.c:7383:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
    keylen = 16;
    ~~~~~~~^~~~
../drivers/net/wireless/realtek/rtl8192cu/os_dep/linux/ioctl_linux.c:7384:3: note: here
   default:
   ^~~~~~~
../drivers/net/wireless/realtek/rtl8192cu/os_dep/linux/ioctl_cfg80211.c: In function ‘set_group_key’:
../drivers/net/wireless/realtek/rtl8192cu/os_dep/linux/ioctl_cfg80211.c:822:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
    keylen = 16;
    ~~~~~~~^~~~
../drivers/net/wireless/realtek/rtl8192cu/os_dep/linux/ioctl_cfg80211.c:823:3: note: here
   default:
   ^~~~~~~

None of them appear to be a real issue but it is trivial to make the
warnings go away.

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>