i2c: tegra: remove unused member variable

Remove unused member variable "iomem" of the
i2c device structure.

This variable becomes unused when converted all allocation
to devm_* in following change:
    i2c: tegra: make all resource allocation through devm_*

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>