MIPS: pmcs-msp71xx: Remove unused addr variable

The addr variable in prom_free_prom_memory() has been unused since
commit b3c948e2 ("MIPS: msp: Record prom memory"), causing a warning
& build failure due to -Werror. Remove the unused variable.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: b3c948e2 ("MIPS: msp: Record prom memory")
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-mips@vger.kernel.org