i2c: mxs: Pass the i2c_adapter around

This patch permeates the struct i2c_adapter throughout the driver,
so that it can be used to determine which adapter should be used.
Note that the driver still supports only one adapter. Note that
the patch does shuffle mxs_i2c_set_bus_speed() to the top of the
driver, but the function remains unchanged.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Heiko Schocher <hs@denx.de>