HSI: ssi_protocol: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: arm):

drivers/hsi/clients/ssi_protocol.c: In function ‘ssip_set_rxstate’:
drivers/hsi/clients/ssi_protocol.c:291:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (atomic_read(&ssi->tx_usecnt))
      ^
drivers/hsi/clients/ssi_protocol.c:294:2: note: here
  case RECEIVING:
  ^~~~

drivers/hsi/clients/ssi_protocol.c: In function ‘ssip_keep_alive’:
drivers/hsi/clients/ssi_protocol.c:466:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (atomic_read(&ssi->tx_usecnt) == 0)
       ^
drivers/hsi/clients/ssi_protocol.c:472:3: note: here
   case SEND_IDLE:
   ^~~~

Notice that, in this particular case, the code comment is
modified in accordance with what GCC is expecting to find.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>