powerpc/pmac/low_i2c.c: three minor problems

Fix minor nits found by cppcheck

[./arch/powerpc/platforms/powermac/low_i2c.c:594]: (style) The scope of the variable chans can be reduced
[./arch/powerpc/platforms/powermac/low_i2c.c:594]: (style) The scope of the variable i can be reduced
[./arch/powerpc/platforms/powermac/low_i2c.c:1260]: (style) Redundant condition. It is safe to deallocate a NULL pointer

Signed-off-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>