ffmpeg: don't pass --enable-mipsfpu on non-MIPS platform

ffmpeg: don't pass --enable-mipsfpu on non-MIPS platform

The current logic to pass the --{enable,disable}-mipsfpu option is:

ifeq ($(BR2_MIPS_SOFT_FLOAT),y)
FFMPEG_CONF_OPTS += --disable-mipsfpu
else
FFMPEG_CONF_OPTS += --enable-mipsfpu
endif

In practice, this means that on MIPS soft-float, --disable-mipsfpu is
passed, and that in *all* other cases, --enable-mipsfpu is passed,
including if we are *not* targetting the MIPS architecture.

Even though this doesn't seem to cause any problem, it is a bit weird to
see --enable-mipsfpu when you're building ffmpeg for a non-MIPS
architecture, so we better fix this by enclosing the MIPS-related
options in a MIPS condition.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>