ath10k: use devm_clk_get() instead of clk_get()

Use the managed variant of clk_get() to simplify the failure path
and the .remove callback.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>