Drop unnecessary check of TARGET_PHYS_ADDR_SPACE_BITS

For all our PPC targets the physical address space is at least
36 bits, so drop an unnecessary preprocessor conditional check
on TARGET_PHYS_ADDR_SPACE_BITS (erroneously introduced as part
of the change from target_phys_addr_t to hwaddr). This brings
this bit of code into line with the way we handle the other
cases which were originally checking TARGET_PHYS_ADDR_BITS in
order to avoid compiler complaints about overflowing a 32 bit type.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
