powerpc/math-emu: Remove -w build flag and fix warnings

As reported by Nathan, the module_init() macro was not taken into
account because the header was missing. That means spe_mathemu_init()
was never called.

This should have been detected by gcc at build time, but due to
'-w' flag it went undetected.

Removing that flag leads to many warnings hence errors.

Fix those warnings then remove the -w flag.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/2663961738a46073713786d4efeb53100ca156e7.1662134272.git.christophe.leroy@csgroup.eu