rtc: brcmstb-waketimer: fix error handling in brcmstb_waketmr_probe()

brcmstb_waketmr_probe() does not disable timer->clk on error paths.

Found by Linux Driver Verification project (linuxtesting.org).

Fixes: c4f07ece ("rtc: brcmstb-waketimer: Add Broadcom STB wake-timer")
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>