i2c: mpc: Use atomic read and fix break condition

Maxime points out that the polling code in mpc_i2c_isr should use the
_atomic API because it is called in an irq context and that the
behaviour of the MCF bit is that it is 1 when the byte transfer is
complete. All of this means the original code was effectively a
udelay(100).

Fix this by using readb_poll_timeout_atomic() and removing the negation
of the break condition.

Fixes: 4a8ac5e4

 ("i2c: mpc: Poll for MCF")
Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Wolfram Sang <wsa@kernel.org>