i2c: mxs: Use i2c_set_bus_speed() in i2c_init()

Use i2c_set_bus_speed() in i2c_init() within the mxs i2c driver
to avoid duplication of code.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>