[non BE] Resolve "OSBA bitrate switching crashes with MONO output"

• Related issues: #774 (closed)
• Requested reviewers: @fotopoulou

Reason why this change is needed

crash with OSBA to mono output when switching from pre-rendered to discrete mode

Description of the change

open the ism to mono dowmix renderer in ivas_sba_dec_reconfigure

Affected operating points

• OSBA to mono output with bitrate switching is affected
• Produced bitstream is BE compared to previous state, no encoder changes
• output is not BE, decoding fails without this MR

Related to #774 (closed)