clk: bcm: Fix 'maybe-uninitialized' warning in bcm2835_clock_choose_div_and_prate()

best_rate is reported as potentially uninitialized by gcc.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: 155e8b3b

 ("clk: bcm: Support rate change propagation on bcm2835 clocks")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>