scmi: Make transports report the effective length of the received messages

Once a message is received that fits into the RX buffer, update the
msg.rx_len field with the effective length of the message received.

Tested on: Arm Morello Board
Reviewed by: andrew
Sponsored by: Arm Ltd
Pull Request: https://reviews.freebsd.org/D47420
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>