signal/mips: Remove FPE_FIXME usage from mips

Ralf Baechle recently merged a change to stop explicitly handling this
case on mips, as the case is impossible.  Unfortunately when the code
in force_fcr31_sig to not use siginfo but a simple interger variable gcc
starts warning about an unitialized variable.

So add a return statement so gcc knows the code never attempts to use
the impossible value.

Cc: Ralf Baechle <[email protected]>
Signed-off-by: "Eric W. Biederman" <[email protected]>