i2c-bfin-twi: Register adapter with a specific bus number

All the users of this driver explicitly specify the I2C bus numbers
to be used in their platform data. Make the driver respect that.

Signed-off-by: Kalle Pokki <kalle.pokki@eke.fi>
Cc: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>