clocksource/drivers/em_sti: Fix variable declaration in em_sti_probe

'irq' and 'ret' are variables of the same type and there is no
need to use two lines.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191221173027.30716-3-tiny.windzz@gmail.com