Bluetooth: qca: simplify WCN399x NVM loading

The WCN399x code has two separate cases for loading the NVM data. In
preparation to adding support for WCN3950, which also requires similar
quirk, split the "variant" to be specified explicitly and merge two
snprintfs into a single one.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>