Few functions in ivas_cov_smooth.c converted to fxd

Converted ivas_calculate_update_factor and ivas_set_up_cov_smoothing functions to fixed point.

pytests results:
353 failed, 229 passed, 381 skipped

MLD scores:
Original inputs: (20 cases > 5)

5.065071 5.371918 5.43706 5.795138 6.303811 6.405441 6.405441 6.435398 6.565653 6.670436 7.566347 9.193686 10.476076 10.830501 10.853147 10.854506 13.111792 13.111792 13.618346 13.618346

+10dB scaled inputs: (46 cases > 5)
5.255993 5.374277 5.380199 5.724338 5.724338 5.788807 5.84246 5.873578 6.009849 6.016915 6.326981 6.443156 6.654574 6.666913 6.831288 7.34617 7.405177 7.45216 7.78002 7.856937 8.026753 8.149709 9.172699 9.38772 9.407003 9.407003 9.480333 9.694523 10.140915 10.500914 10.810769 11.14054 11.14054 11.200235 11.334195 12.845577 15.199893 15.358177 16.396973 16.396973 16.975739 17.258497 19.3745 19.761221 27.127409 28.68569

-10dB scaled inputs: (12 cases > 5)
5.02216 5.074999 5.964681 5.9852 6.009333 6.297444 7.091984 7.091984 7.117018 7.117018 15.455975 15.455975