Added visibility to the UIPC_Read() errors

Changed some of the BTIF_TRACE_EVENT() log messages
to BTIF_TRACE_WARNING() so we can get better visibility into the
UIPC_Read() errors.

Also, removed UIPC_ReadBuf() and UIPC_SendBuf(), because they are
not implemented / used.

Change-Id: Iccc6a4d73dfe3c3e57f21c538891d7cb429f11ac