i2c: i801: add helper i801_restore_regs

In few places relevant registers are reset to their initial value on
driver load. Factor this out to new helper i801_restore_regs to avoid
code duplication.
Even though no actual problems are known, this patch may contribute
to avoiding potential issues by:
- restoring register values also in the error path of i2c_add_adapter
- making restoring registers the last step (especially in i801_remove)

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Acked-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>