OMAP3: Avoid re-write to PRM_CLKSRC_CTRL

In function get_osc_clk_speed(), do not change/ update
the divider for SYS_CLK as it can has cascading effect
on the other derived clocks.

Sudden change in divider value can lead to inconsistent
behavior in the system - often leading to crashes.

The problem was found when working with OMAP3EVM using
DM3730 processor card.

The patch has been tested with OMAP3530 on OMAP3EVM as
well

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Hiremath Vaibhav <hvaibhav@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>