clk: at91: generated: set audio_pll_allowed in at91_clk_register_generated()

Set gck->audio_pll_allowed in at91_clk_register_generated. This makes it
easier to do it from code that is not parsing device tree.

Also, this fixes an issue where the resulting clk_hw can be dereferenced
before being tested for error.

Fixes: 1a1a36d7 ("clk: at91: clk-generated: make gclk determine audio_pll rate")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>