Slow path for iget should expect return in core reg

Slow path for iget invokes the C implementation.
In all cases the C function returns the result in core reg.
So implementation should expect the result in core reg
independent on whether it is fp or not.

Change-Id: I57fb0e684c38af22316398d8071f087bd4bd253c
Signed-off-by: Serguei Katkov <serguei.i.katkov@intel.com>