i2c: mxs: correctly setup speed for non devicetree

Commit cd4f2d4a (i2c: mxs: Set I2C timing registers for mxs-i2c) only
covered the case for devicetree and made platform_data based boards
bail out with -EINVAL. Correctly support the latter one, too.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>