fix(st-clock): correctly check ready bit

The function clk_oscillator_wait_ready() was wrongly checking the set
bit and not the ready bit. Correct that by using osc_data->gate_rdy_id
when calling _clk_stm32_gate_wait_ready().

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: Ida58f14d7f0f326b580ae24b98d6b9f592d2d711