i2c: fsl_i2c: fix m68k transferts

This driver is actually used for powerpc and m68k/ColdFire.

On ColdFire SoC's, interrupt flag get not set if IIEN flag (mbcr bit6,
interrupt enabled) is not set appropriately before each transfert.
As a result, the transfert hangs forever waiting for IIEN.
This patch set IIEN before each transfert, while considering this fix
as not harming powerpc arch.

Signed-off-by: Angelo Dureghello <angelo@kernel-space.org>