mfd: wm831x-i2c: Make it explicitly non-modular

The Kconfig currently controlling compilation of this code is:

drivers/mfd/Kconfig:config MFD_WM831X_I2C
drivers/mfd/Kconfig:    bool "Wolfson Microelectronics WM831x/2x PMICs with I2C"

...meaning that it currently is not being built as a module by anyone.

Lets remove the modular code that is essentially orphaned, so that
when reading the driver there is no doubt it is builtin-only.

We explicitly disallow a driver unbind, since that doesn't have a
sensible use case anyway, and it allows us to drop the ".remove"
code for non-modular drivers.

Since module_init was not in use by this code, the init ordering
remains unchanged with this commit.

Also note that MODULE_DEVICE_TABLE is a no-op for non-modular code.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>