scmi: Refactor scmi_request internal logic

Split scmi_request internals into a TX and an RX phase, which will be
called, anyway, in sequence as before when a normal transmission is
requested.

No functional change.

Tested on: Arm Morello Board
Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47425
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>