ab8500-charger: Add UsbLineCtrl2 reference

When the state of USB Charge detection is changed then the calls
use a define for another register in other bank. This change
creates a new define for the correct register and removes the
magic numbers that are present.

Signed-off-by: Marcus Cooper <marcus.xm.cooper@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Hakan BERG <hakan.berg@stericsson.com>
Reviewed-by: Jonas ABERG <jonas.aberg@stericsson.com>