clk: mediatek: return XTAL rate for infrasys get_mux_rate

clk: mediatek: return XTAL rate for infrasys get_mux_rate

We currently return 0 if XTAL rate is requested in get_mux_rate. This
deviates from what is done in get_factor_rate and is totally wrong as it
can cause unwanted results (division by 0 crash)

For infrasys that makes use of CLK_XTAL, assume xtal_rate to be defined
in clk_tree and return the rate when BYPASS_XTAL is not enabled with
clk ID 0 index parents.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>