powerpc/mpc52xx: Properly update irq_desc when set_type() is called.

The MPC5200 PIC driver doesn't correctly update the .status field of
the irq_desc structure when the set_type hook is called.  This patch
adds the required code.

Also cleans up the external IRQ typename field to be something easier
to read (very minor).

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>