lib/meta-flow: introduce a macro, CASE_MFF_REGS, to catch "case MFF_REG<N>:"

Introduce a macro instead for
With this macro, the code is a bit reduced.
test: compile-tested and unit tests passed.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
[blp@nicira.com moved the macro declaration, moved trailing colon from
 macro definition to invocation, adjusted style slightly]
Signed-off-by: Ben Pfaff <blp@nicira.com>