gst-ffmpeg: fix --disable-yasm

gst-ffmpeg: fix --disable-yasm

The gst-ffmpeg build shows the following warning:

configure: WARNING: unrecognized options: --disable-yasm

which means that the following test in configure always fails and
--disable-yasm never gets passed to the embedded ffmpeg build:

'if test "x$disable_yasm" = "xyes"; then'
  embffmpeg_configure_args="$embffmpeg_configure_args --disable-yasm"

commit 4d309730 ['gst-ffmpeg: configure-fix patch used wrong test']
actually fixed the obviously backwards syntax by reversing the test -
prior to that, --disable-yasm would always unconditionally be passed
into the embedded ffmpeg config.

This fixes things so that the variable actually exists and makes the
test meaningful.

(From OE-Core rev: da9515621134c26e54f43b96cdad0c6e6c5876bf)

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>