power: smb135x-charger: fix fast charge current mask

FCC_MASK masks bits [5:0] which does not match the hardware
specification of writing only to bits [4:0] for configuring FCC.
Fix it.

CRs-Fixed: 877479
Change-Id: I6443fd3dbbad75539b250a085750a3cb59b73734
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>
Signed-off-by: Gopal G Goberu <ggober@codeaurora.org>