clocksource/drivers/time-armada-370-xp: Fix the clock reference

While converting the init function to return an error, the wrong clock
was get. This leads to the wrong clock rate and slows down the kernel.
For example, it affects typical boot time:

 - without fix: over 1 minute
 - with    fix: 15 seconds

Tested-by: Stefan Roese <sr@denx.de>
Tested-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 12549e27c63c ("clocksource/drivers/time-armada-370-xp: Convert init function to return error")
Link: http://lkml.kernel.org/r/1471429296-9053-1-git-send-email-daniel.lezcano@linaro.org
[ Refined the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>