mmc: dw_mmc: move host->data_offset init earlier

host->data_offset is initialised at the end of the probe function
depending on the VERID register, and is used for PIO operations. Move
this initialisation earlier, before IRQs or slots are initialised, to be
sure that PIO won't occur prior to host->data_offset being initialised.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>