Blackfin: fix gptimer0 clock source compile warning

Some of the clocksource prototypes were updated, but the gptimer0 func was
missed in the process.  Not a big issue as the argument is ignored, but we
should fix the compile warning anyways.

Signed-off-by: Yi Li <yi.li@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>