mmc: omap_hsmmc: fix return error code for of_get_hsmmc_pdata

of_get_hsmmc_pdata returns a pointer, returning NULL is invalid,
return ERR_PTR for error case.

Signed-off-by: Balaji T K <balajitk@ti.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chris Ball <chris@printf.net>