ab8500-fg: Change current calculation

This patch updates the gas gauge constant for electric current
calculation according to hardware specification.

Signed-off-by: Benoit GAUTHIER <benoit.gauthier-xsig@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Marcus COOPER <marcus.xm.cooper@stericsson.com>
Reviewed-by: Philippe LANGLAIS <philippe.langlais@stericsson.com>
Tested-by: Benoit GAUTHIER <benoit.gauthier-xsig@stericsson.com>