mxs_ocotp: Shift the HBUS divider correctly

When the original HBUS divider value is retrieved in mxs_ocotp_scale_hclk()
for the purpose or restoring it back later, the value is not shifted by the
HBUS divider offset in that register. This is not a problem, since the shift
is zero on all MXS hardware. Add the shift anyway, for completeness and in
case FSL ever decides to re-use this driver on future designs.

Signed-off-by: Chris Smith <chris@zxdesign.info>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>