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: 41691b88

 ("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>