ath10k: move enable_pll_clk call to ath10k_core_start()

There's no reason to have call for enable_pll_clk in ath10k_bmi_start(), move
it to ath10k_core_start() instead. This way it's possible to call
ath10k_bmi_start() from sdio.c during firmware dump creation. And also the
function call is more visible when it's in core.c.

No functional changes, compile tested only.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1597421745-4329-1-git-send-email-kvalo@codeaurora.org