i2c: i2c-qcom-geni: Correct the error handling sequence in I2C GSI mode

In case of error, unmap the dma buffer after cleaning up GSI channels
to ensure GSI transfers are stopped at HW level before dma buffer is
unmapped.

Change-Id: I9e0c51c8b3caec0ba03cada407b67674622e7e13
Signed-off-by: Vipin Deep Kaur <vkaur@codeaurora.org>