g723.1: simplify and fix multiplication overflow

In 16-bit arithmetic, x * 0xffffc is simply x * -4 with extra overflows,
(and the constant was probably meant to be 0xfffc).  Combined with the
shift, this simplifies to -x >> 1.  Finally, clearing the low two bits
with a 32-bit mask and switching to a 32-bit type allows more efficient
code on 32-bit machines.

Signed-off-by: Mans Rullgard <mans@mansr.com>
