slimbus: Scheduling algorithm should ensure non-zero channel-offset

Messaging slots are at the beginning of a slimbus superframe.
A channel cannot start at offset 0 in the superframe to ensure
messaging space gets non-zero slots.
Make sure channel scheduling algorithm leaves some slots for message
space.

Change-Id: I25dd3703b1ae331f4b8017b9ca92410f5811088c
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>