math-emu: Fix thinko in _FP_DIV

In commit 48d6c643

 ("math-emu: Add
support for reporting exact invalid exception") code was added to
set the new FP_EX_INVALID_{IDI,ZDZ} exception flag bits.

However there is a missing break statement for the
_FP_CLS_COMBINE(FP_CLS_INF,FP_CLS_INF) switch case, the
code just falls into _FP_CLS_COMBINE(FP_CLS_ZERO,FP_CLS_ZERO)
which then proceeds to overwrite all of the settings.

Fix by adding the missing break.

Signed-off-by: David S. Miller <davem@davemloft.net>