clk: qcom: fix 16 bit alpha support calculation

The alpha value calculation has been written for 40-bit alpha
values which doesn't work work properly for 16-bit ones. The
alpha value is calculated on the basis of ALPHA_BITWIDTH to make
the computation easy for 40 bit alpha. After calculating the 32
bit alpha, it is converted to 40 bit alpha by making lower bits
zero. But if actual alpha register width is less than
ALPHA_BITWIDTH, then the actual width can be used for
calculation. This also means, during the 40 bit alpha pll set
rate path, the lower alpha register is not configured

Change the code to calculate the rate and register values from
'alpha_width' instead of hard-coding it so that it can work for
the different widths that are supported.

Signed-off-by: Abhishek Sahu <absahu@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>