microblaze: Fix return value from xilinx_timer_init

The patch
"clocksource/drivers/microblaze: Convert init function to return error"
(sha1: 05864217)
introduced return value and this one was forgetten to convert.

This patch also remove compilation warning:
arch/microblaze/kernel/timer.c: In function 'xilinx_timer_init':
arch/microblaze/kernel/timer.c:262:3: warning: 'return' with no value,
 in function returning non-void [-Wreturn-type]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>