sparc: Treat the version field in the FPU control word as reserved

The FSR version field is read-only and might be non-zero.

This allows math/test-fpucw* to correctly pass when the version is
non-zero.

Signed-off-by: Daniel Cederman <cederman@gaisler.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>