net: ipa: encapsulate decision about firmware load

The GSI layer used for IPA requires firmware to be loaded.

Currently either the AP or the modem loads the firmware,
distinguished by whether the "modem-init" Device Tree
property is defined.

Some newer systems implement a third option.  In preparation for
that, encapsulate the code that determines how the GSI firmware
gets loaded in a new function, ipa_firmware_loader().

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>