qtnfmac: fix core attach error path in pcie backend

Report that firmware is up and running only for successful firmware
download. Simplify qtnf_pcie_fw_boot_done: modify error path so that
no need to pass firmware dowload result to this function. Finally,
do not create debugfs entries if firmware download succeeded,
but core attach failed.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>