i2c: s3c2410: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/i2c/busses/i2c-s3c2410.c: In function 'i2c_s3c_irq_nextbyte':
drivers/i2c/busses/i2c-s3c2410.c:431:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (i2c->state == STATE_READ)
      ^
drivers/i2c/busses/i2c-s3c2410.c:439:2: note: here
  case STATE_WRITE:
  ^~~~

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

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>