clk: si5351: Convert __clk_get_name() to clk_hw_get_name()

This driver uses __clk_get_name() when it's dealing with struct
clk_hw pointers. Use the simpler form so that we can get rid of
the clk member in struct clk_hw one day.

Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>