re-look into c_sw_pll channel comms

[shuchitak]

Communication between clockgen and sw_pll_task over the c_sw_pll channel could be improved, as mentioned in the comments on #508.
Look into either optimising it, for example,

outuint(c_sw_pll, DISABLE_SDM); /* Resets SDM */
outct(c_sw_pll, XS1_CT_END);
outuint(c_sw_pll, selected_mclk_rate);
outct(c_sw_pll, XS1_CT_END);

could be

outuint(c_sw_pll, DISABLE_SDM); /* Resets SDM */
outuint(c_sw_pll, selected_mclk_rate);
outct(c_sw_pll, XS1_CT_END); 

Or, making it safer by using the xc :> operator,

depending on what's required when communicating over this channel.