In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

In file included from drivers/net/wimax/i2400m/debug-levels.h:30,
                 from drivers/net/wimax/i2400m/control.c:86:
drivers/net/wimax/i2400m/control.c: In function ‘i2400m_report_tlv_system_state’:
./include/linux/wimax/debug.h:200:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
 do {         \
    ^
./include/linux/wimax/debug.h:404:36: note: in expansion of macro ‘_d_printf’
 #define d_printf(l, _dev, f, a...) _d_printf(l, "", _dev, f, ## a)
                                    ^~~~~~~~~
drivers/net/wimax/i2400m/control.c:354:3: note: in expansion of macro ‘d_printf’
   d_printf(1, dev, "entering BS-negotiated idle mode\n");
   ^~~~~~~~
drivers/net/wimax/i2400m/control.c:355:2: note: here
  case I2400M_SS_DISCONNECTING:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>