arch: m68k: Use existing CONFIG_MCFTMR instead of CFG_MCFTMR

There is an existing CONFIG_MCFTMR Kconfig symbol,
use it and drop all other instances of CFG_MCFTMR.
This duality is likely a result of bogus conversion
to Kconfig.

Fixes: 7ff7b46e ("m68k: rename CONFIG_MCFTMR to CFG_MCFTMR")
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>