Bug in fofb_sys_id_pkg.f_signed_saturate

There's a bug in https://github.com/lnls-dig/fofb-ctrl-gw/blob/2bf44cd5468c1bab727aa1d9276751c3825a7506/hdl/modules/fofb_sys_id/fofb_sys_id_pkg.vhd#L119C1-L119C41. This clause should detect sign extensions, but it only does correctly if x's width is 1 bit larger than trunc_x_len.

This bug doesn't affect prbs_bpm_pos_distort core because there we're shrinking from 33 -> 32 bits, but it does affect prbs_bpm_sp_distort (17 -> 15 bits). If the distorted set point before saturation is smaller than -32768 or greater than 32767, no saturation will be performed and the function would simply truncate the 15LSBs of it.