MIPS: Remove obsolete MIPS checks for DST node "chosen@0"

As there is precious little left in any DTS files referring to the
node "/chosen@0" as opposed to "/chosen", remove the two checks for
the former node name.

[paul.burton@mips.com:
  The modified yamon-dt code only operates on
  arch/mips/boot/dts/mti/sead3.dts right now, and that uses chosen
  rather than chosen@0 anyway, so this should have no behavioural
  effect.]

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20131/
Cc: linux-mips@linux-mips.org