btm_sco_hfp_hal: Change get_packet_size() return type to size_t

The users of get_packet_size() (bluetooth::audio::sco::(wbs|swb)::init)
actually accept the packet size as size_t, and in kernel wbs_pkt_len is
also defined as an unsigned value. There's no reason to define its
return type as signed int.

Bug: 327547486
Tag: #floss
Test: mmm packages/modules/Bluetooth
Flag: EXEMPT, only type change and get_packet_size() always returns 0 on
      Android.

Change-Id: I658014a992308cd1f95fa74bda07cecc99348e1a