net: ipa: retry modem stop if busy

The IPA driver remove callback, ipa_remove(), calls ipa_modem_stop()
if the setup stage of initialization is complete.  If a concurrent
call to ipa_modem_start() or ipa_modem_stop() has begin but not
completed, ipa_modem_stop() can return an error (-EBUSY).

The next patch adds a driver shutdown callback, which will simply
call ipa_remove().  We really want our shutdown callback to clean
things up.  So add a single retry to the ipa_modem_stop() call in
ipa_remove() after a short (millisecond) delay.  This offers no
guarantee the shutdown will complete successfully, but we'll at
least try a little harder before giving up.

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