ath6kl: handle firmware names more dynamically

Currently ath6kl has just hardcoded paths to each firmware file. Change
this more dynamic by separating the the directory and file name from each
other. That way it's easier to dynamically create full paths to firmware and
code looks better. And now it's possible to remove a function needed by
devicetree code.

While at it add a structure inside struct ath6kl_hw to contain all
firmware names. I deliberately omitted board file support as
those will be handled later.

This is needed for firmware API 3.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>