ARM: Fix decoding of Neon forms of VCVT between float and fixed point

ARM: Fix decoding of Neon forms of VCVT between float and fixed point

Fix errors in the decoding of the Neon forms of fixed-point VCVT:
 * fixed-point VCVT is op 14 and 15, not 15 and 16
 * the fbits immediate field was being misinterpreted
 * the sense of the to_fixed bit was inverted

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Nathan Froyd <froydnj@codesourcery.com>