gpio/vt8500: memory cleanup missing

This driver is missing a .remove callback, and the fail path on
probe is incomplete.

If an error occurs in vt8500_add_chips, gpio_base is not unmapped.
The driver is also ignoring the return value from this function so
if a chip fails to register it completes as successful.

Replaced pr_err with dev_err in vt8500_add_chips since the device is
available.

There is also no .remove callback defined so the function is added.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>