ath10k: use firmware_request_nowarn() to load firmware

This reduces the unnecessary spew when trying to load optional firmware:
"Direct firmware load for ... failed with error -2"

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>