firmware: arm_scmi: drop unused `con_priv` structure member

Initially con_priv was supposedly used for transport specific data when
the SCMI driver had an abstraction to communicate with different mailbox
controllers. But after some discussions, the idea was dropped but this
variable slipped through the cracks.

This patch gets rid of this unused variable.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>