virt: gunyah: rm_core: Remove current_recv_buffer tracking

current_recv_buffer tracks the current end of an ongoing message from
resource manager. It can be easily tracked by "conection->recv_buff +
connection->recv_buff_size". Do that instead to make code clearer.

Change-Id: I191dbdbf39c802868e1c6a262ef5420c02f127b2
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>