spi: stm32: Fix error handling in stm32_spi_probe()

clk_get_rate() is below clk_prepare_enable(), so
its error should lead to goto err_clk_disable, not to err_master_put.

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

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Reviewed-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>