i3c: master: dw-i3c-master: mark expected switch fall-through

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

This patch fixes the following warning:

drivers/i3c/master/dw-i3c-master.c: In function ‘dw_i3c_master_bus_init’:
drivers/i3c/master/dw-i3c-master.c:603:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (ret)
      ^
drivers/i3c/master/dw-i3c-master.c:605:2: note: here
  case I3C_BUS_MODE_PURE:
  ^~~~

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>
Acked-by: Vitor Soares <vitor.soares@synopsys.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>