MIPS: pmcs-msp71xx: Add missing MAX_PROM_MEM definition

Commit b3c948e2 ("MIPS: msp: Record prom memory") introduced use of
a MAX_PROM_MEM value but didn't define it. A bounds check in
prom_meminit() suggests its value was supposed to be 5, so define it as
such & adjust the bounds check to use the macro rather than a magic
number.

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