MIPS: math-emu: <MAXA|MINA>.<D|S>: Fix cases of both infinite inputs

MIPS: math-emu: <MAXA|MINA>.<D|S>: Fix cases of both infinite inputs

Fix the value returned by <MAXA|MINA>.<D|S> fd,fs,ft, if both inputs
are infinite. The previous implementation returned always the value
contained in ft in such cases. The correct behavior is specified
in Mips instruction set manual and is as follows:

    fs    ft        MAXA     MINA
  ---------------------------------
    inf   inf        inf      inf
    inf  -inf        inf     -inf
   -inf   inf        inf     -inf
   -inf  -inf       -inf     -inf

A relevant example:

MAXA.S fd,fs,ft:
  If fs contains +inf, and ft contains -inf, fd is going to contain
  +inf (without this patch, it used to contain -inf).

Fixes: 007b01684dd6 ("MIPS: math-emu: Add support for the MIPS R6 MAX{, A} FPU instruction")
Fixes: 8c53dc70e4b8 ("MIPS: math-emu: Add support for the MIPS R6 MIN{, A} FPU instruction")
Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>
Signed-off-by: Goran Ferenc <goran.ferenc@imgtec.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Cc: Bo Hu <bohu@google.com>
Cc: Douglas Leung <douglas.leung@imgtec.com>
Cc: Jin Qian <jinqian@google.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Petar Jovanovic <petar.jovanovic@imgtec.com>
Cc: Raghu Gandham <raghu.gandham@imgtec.com>
Cc: <stable@vger.kernel.org> # 4.3+
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16884/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>