net: ipa: ipa_stop() does not return an error

In ipa_modem_stop(), if the modem netdev pointer is non-null we call
ipa_stop().  We check for an error and if one is returned we handle
it.  But ipa_stop() never returns an error, so this extra handling
is unnecessary.  Simplify the code in ipa_modem_stop() based on the
knowledge no error handling is needed at this spot.

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