mxc_i2c: remove setting speed at each start

Other then being very weird, this code was also wrong.
For example, say I set speed to 100K. I'll read back the speed
as 85937. But the speed is really 85937.5, so we I reset
the speed to 85937, I'll get 73660.7. After a couple of transactions
my speed is now exactly 68750 so it will remain there.

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