i2c: efm32: fix a failure path in efm32_i2c_probe()

There is the only failure path in efm32_i2c_probe(),
where clk_disable_unprepare() is missed.

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

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Fixes: 1b5b2371 ("i2c: efm32: new bus driver")
Cc: stable@kernel.org