x86: Correct mrccache find_next_mrc_cache() calculation

This should take account of the end of the new cache record since a record
cannot extend beyond the end of the flash region. This problem was not
seen before due to the alignment of the relatively small amount of MRC
data.

But with Apollo Lake the MRC data is about 45KB, even if most of it is
zeroes.

Fix this bug and update the parameter name to be less confusing.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>