power: smb5: Fix fast charge current step size

The current CHGR_FAST_CHARGE_CURRENT_CFG step size is
is 25000uA, while the right value per spec should be
50000uA.

Correct it.

CRs-Fixed: 2237758
Change-Id: I6c201fb10ec6eb0ffc70c14e83d69e7fff7ec121
Signed-off-by: Harry Yang <harryy@codeaurora.org>