powerpc: Explicitly reverse bytes when checking for byte reversal

Sparse reports an invalid endian cast here. The code is written for
big endian platforms, so le32_to_cpu() acts as a byte reversal.

This file is checked by sparse on a little endian build though, so
replace the reverse function with the dedicated swab32() function to
better express the intent of the code.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231011053711.93427-4-bgray@linux.ibm.com