ath11k: move WCN6750 firmware to the device-specific subdir

ath11k: move WCN6750 firmware to the device-specific subdir

As demonstrated by the inability to use existing WCN6750 firmware on
QCM6490 / QCS6490 devices, such firmware files are device-specific. As a
preparation to adding more device-specific firmware, move existing
firmware to the sc7280/ subdir, squashing it in proceess. Squashing it
simplifies loading (as it requires just a single file access / userspace
call) and also it requires just a single compatibility symlink instead
of ten.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>