max8997_charger: Fix unsigned value for less than zero

The 'val' is a 'unsigned char', so it is never less than zero.

Signed-off-by: Jonghwan Choi <jhbird.choi@gmail.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>