staging: r8188eu: Add "fallthrough" statement to quiet compiler

Compiler gcc11, and possibly others, emit a warning when a fall-through
case is found in a switch statement. Add a "fallthrough" statement to
eliminate this warning.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Link: https://lore.kernel.org/r/20210729170930.23171-7-Larry.Finger@lwfinger.net


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>