global_data: remove unused mxc_i2c specific field

The srdata field is unused since commit 71204e95 ("i2c: mxc:
refactor i2c driver and support dm").

Cc: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Peng Fan <peng.fan@nxp.com>