sim: ppc: fix implicit enum conversion

This code tries to use attach_type enums as hw_phb_decode, and while
they're setup to have compatible values, the compiler doesn't like it
when the cast is missing.  So cast it explicitly and then use that.

sim/ppc/hw_phb.c:322:28: error:
	implicit conversion from enumeration type 'attach_type'
	(aka 'enum _attach_type') to different enumeration type
	'hw_phb_decode' [-Werror,-Wenum-conversion]