powerpc/mpic: Fix use of "flags" variable in mpic_alloc()

The mpic_alloc() function takes a "flags" parameter and assigns it into
the mpic->flags variable fairly early on, but several later pieces of
code detect various device-tree properties and save them into the
"mpic->flags" variable (EG: "big-endian" => MPIC_BIG_ENDIAN).

Unfortunately, a number of codepaths (including several which test the
flag MPIC_BIG_ENDIAN!) test "flags" instead of "mpic->flags", and get
wrong answers as a result.

Consolidate the device-tree flag tests early in mpic_alloc() and change
all of the checks after "mpic->flags" is init'ed to use "mpic->flags".

[BenH: Fixed up use of mpic->node before it's initialized]

Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>