MIPS: BCM47xx: Fix nvram_getenv return value.

Nvram_getenv should behave like cfe_getenv. cfe_getenv returns 0 on
success and -9 if the value was not found. If the input was wrong -8
will be returned by cfe_getenv.  Change nvram_getenv to do the same.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Waldemar Brodkorb <wbx@openadk.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1520/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>