i2c-bfin-twi: move setup to the earlier subsys initcall

Some systems using this bus sometimes have very basic devices such as
regulators on the bus, so the I2C bus master needs to be loaded early.
This also matches the behavior of many other I2C bus master drivers.
Therefore initialize via subsys_initcall().

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>