powerpc/fadump: Annotate endianness cast with __force

Sparse reports an endianness error with the else case of

  val = (cpu_endian ? be64_to_cpu(reg_entry->reg_val) :
         (u64)(reg_entry->reg_val));

This is a safe operation because the code is explicitly working with
dynamic endianness, so add the __force annotation to tell Sparse to
ignore it.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231011053711.93427-13-bgray@linux.ibm.com