dt-bindings: net: qcom,ipa: remove an unnecessary restriction

Commit d8604b20 ("dt-bindings: net: qcom,ipa: add firmware-name
property") added a requirement for a "firmware-name" property that
is more restrictive than necessary.

If the AP loads GSI firmware, the name of the firmware file to use
may optionally be provided via a "firmware-name" property.  If the
*modem* loads GSI firmware, "firmware-name" doesn't need to be
supplied--but it's harmless to do so (it will simply be ignored).

Remove the unnecessary restriction, and allow "firware-name" to be
supplied even if it's not needed.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>