rpmsg: slatecom: maintain rx_size to read

For cmd close_ack or open request where rx_size is being
incrementing with respect to offset might lead to out of
bound read from rx_data.

Decrease rx_size as we process commands.

Change-Id: I492eadcbebb78386fc20f744eb9ad8db4a2914fc
Signed-off-by: Kaushal Hooda <quic_khooda@quicinc.com>