clk: bcm2835: Avoid overwriting the div info when disabling a pll_div clk

bcm2835_pll_divider_off() is resetting the divider field in the A2W reg
to zero when disabling the clock.

Make sure we preserve this value by reading the previous a2w_reg value
first and ORing the result with A2W_PLL_CHANNEL_DISABLE.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: 41691b8862e2 ("clk: bcm2835: Add support for programming the audio domain clocks")
Cc: <stable@vger.kernel.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

drivers/clk/bcm/clk-bcm2835.c

@@ -751,7 +751,9 @@ static void bcm2835_pll_divider_off(struct clk_hw *hw)
 	cprman_write(cprman, data->cm_reg,
 		     (cprman_read(cprman, data->cm_reg) &
 		      ~data->load_mask) | data->hold_mask);
-	cprman_write(cprman, data->a2w_reg, A2W_PLL_CHANNEL_DISABLE);
+	cprman_write(cprman, data->a2w_reg,
+		     cprman_read(cprman, data->a2w_reg) |
+		     A2W_PLL_CHANNEL_DISABLE);
 	spin_unlock(&cprman->regs_lock);
 }