[MIPS] PNX8550: Cleanup proc code.

Here's a slightly cleaner way of creating the /proc structure for the
pnx8850.  mostly, it creates a directory with default mode 555, since the
one you're creating is mode 444, which is somewhat unusual for a directory
under /proc.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>