fsl_i2c.c: Fix warning on gcc-6.x

With gcc-6.x we see:
drivers/i2c/fsl_i2c.c:86:3: warning: ‘fsl_i2c_speed_map’ defined but not
used [-Wunused-const-variable=]

The easy way to fix this is that since we only use fsl_i2c_speed_map at
all on __M68K__ move the existing guards around slightly.

Reported-by: Thomas Schaefer <Thomas.Schaefer@kontron.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Heiko Schocher <hs@denx.de>