mips: Fix the increased mem parameter size

The added "mem=size@start" parameter actually corresponds to
"crashkernel=YM@XM", but 1 byte is missing when calculating
the size, so 1 byte should be added.

For example, when using crashkernel=108M@64M (110592K@65536K):
Without this patch:
	the mem parameter added is: mem=110591K@65536K

With this patch:
	the mem parameter added is: mem=110592K@65536K

Fixes: 0eac6405 ("kexec: mips: Fix mem parameters")

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Simon Horman <horms@verge.net.au>