power: pm8921-bms: Fix variable naming

The max_fcc_learning_cycles variable actually indicates
the minimum number of FCC cycles required to update a new FCC,
name it accordingly.

Change-Id: I510d3a05eed736a9ee560c936faa20ed2d3c5bd1
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>