pinctrl: imx27: fix offset calculation in imx_read_2bit

The offset for the 2bit register calculate wrong, this patch
fixes the problem. The debugfs printout for oconf, iconfa, iconfb
now shows the real values.

Cc: stable@vger.kernel.org
Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Reviewed-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>