MIPS: fw/arc: Remove unused addr variable

The addr variable in prom_free_prom_memory() has been unused since
commit 0df10076 ("MIPS: fw: Record prom memory"), leading to a
compiler warning:

  arch/mips/fw/arc/memory.c:163:16:
    warning: unused variable 'addr' [-Wunused-variable]

Fix this by removing the unused variable.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: 0df10076 ("MIPS: fw: Record prom memory")
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-mips@vger.kernel.org