i2c-designware: Move checking of IP core version to i2c_dw_init()

Move checking IP core version to i2c_dw_init() in preparation for
splitting i2c-designware.c into core and bus specific portions.

Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>