powerpc32: don't adjust unmoved stack pointer in csum_partial_copy_generic() epilogue

A recent change to the checksum code removed usage of some extra
arguments, alongside with storage on the stack for those, and the stack
pointer no longer needed to be adjusted in the function prologue.

But a left over subtraction wasn't removed in the function epilogue,
causing the function to return with the stack pointer moved 16 bytes
away from where it should have.  This corrupted local state and lead to
weird crashes.

This simply removes the leftover instruction from the epilogue.

Fixes: 70d65cd5 ("ppc: propagate the calling conventions change down to csum_partial_copy_generic()")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>