mxc_i2c: clear i2sr before waiting for bit

Let's clear the sr register before waiting for
bit to be set, instead of clearing it after
hardware sets it. No real operational difference here,
but allows combining of i2c_imx_trx_complete and
i2c_imx_bus_busy in later patches.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Marek Vasut <marex@denx.de>