clocksource: sh_mtu2: Split channel setup to separate function

Move the channel setup code from sh_mtu2_setup to a new
sh_mtu2_setup_channel function and call it from sh_mtu2_setup.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Wolfram Sang <wsa@sang-engineering.com>