MIPS/BFD: Use helper function for LO relocation sign-extension

A calculation for LO relocations has been recently fixed with commit
ce08b3bb19b3 ("MIPS/BFD: Fix RELA handling of borrow in the generic
linker"), however it was missed that for the updated arithmetic we
already have a helper function available, `_bfd_mips_elf_sign_extend'.

Replace the open-coded statement then with an equivalent call to said
function.  No functional change.