msm: ipa: check that ipa_qmi_ctx is not null before accessing it

When performing ipa_wwan_remove, it is possible that the
IPA QMI initialization sequence hasn't been invoked, causing
the context to be uninitialized.

Change-Id: Id18c5dd50b6f6a659f87c8008132f339f47a8f1d
Acked-by: David Arinzon <darinzon@qti.qualcomm.com>
Signed-off-by: Sivan Reinstein <sivanr@codeaurora.org>