clk: bcm: rpi: Assign ->num before accessing ->hws

In the Linux kernel, the following vulnerability has been resolved:

clk: bcm: rpi: Assign ->num before accessing ->hws

Commit f316cdff8d67 ("clk: Annotate struct clk_hw_onecell_data with
__counted_by") annotated the hws member of 'struct clk_hw_onecell_data'
with __counted_by, which informs the bounds sanitizer about the number
of elements in hws, so that it can warn when hws is accessed out of
bounds. As noted in that change, the __counted_by member must be
initialized with the number of elements before the first array access
happens, otherwise there will be a warning from each access prior to the
initialization because the number of elements is zero. This occurs in
raspberrypi_discover_clocks() due to ->num being assigned after ->hws
has been accessed:

UBSAN: array-index-out-of-bounds in drivers/clk/bcm/clk-raspberrypi.c:374:4
index 3 is out of range for type 'struct clk_hw *[] __counted_by(num)' (aka 'struct clk_hw *[]')

Move the ->num initialization to before the first access of ->hws, which
clears up the warning.