cxl/vphb.c: Use phb pointer after NULL check

static Anlaysis detected below error:-
(error) Possible null pointer dereference: phb

So, Use phb after NULL check.

Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>