net: ipa: optionally define firmware name via DT

net: ipa: optionally define firmware name via DT

IPA initialization includes loading some firmware.  This step is
done either by the modem or by the AP under Trust Zone.  If the
AP loads firmware, the name of the firmware file is currently
hard-coded ("ipa_fws.mdt").

Add the ability to specify the relative path of the firmware file to
use in a property in the Device Tree IPA node.  If the property is
not found (or if any other error occurs attempting to get it), fall
back to using a default relative path.

Use the "old" fixed name as the default.  Rename the symbol that
represents this default to emphasize its purpose.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>