MIPS: Use proper kernel-doc Return keyword

For reference:
* https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html#function-documentation

Fix non-fatal warning:

arch/mips/kernel/branch.c:418: warning: Excess function parameter 'returns' description in '__compute_return_epc_for_insn'

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Maciej W. Rozycki <macro@mips.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18031/
[jhogan@kernel.org: Expand subject slightly]
Signed-off-by: James Hogan <jhogan@kernel.org>