i2c-qcom-geni: Handle corner cases in GSI transfer

Add check to handle if the IOMMU buffer mapping fails,
also make sure to unmap buffer in case of prep_slave_sg failure.

Change-Id: I926a6ce5b10a3c7f29210cba5179cc795ec0c1b4
Signed-off-by: Shrey Vijay <shreyv@codeaurora.org>