usb: host: fhci-hcd: annotate PIPE_CONTROL switch case with fallthrough

usb: host: fhci-hcd: annotate PIPE_CONTROL switch case with fallthrough

After this was made buildable for something other than PPC32, kbuild
starts warning

drivers/usb/host/fhci-hcd.c:398:8: warning: this statement may fall
through [-Wimplicit-fallthrough=]

I don't know this code, but from the construction (initializing size
with 0 and explicitly using "size +=" in the PIPE_BULK case) I assume
that fallthrough is indeed intended.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Li Yang <leoyang.li@nxp.com>
Link: https://lore.kernel.org/r/20200213085401.27862-1-linux@rasmusvillemoes.dk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>