sam9x60_frac_pll_compute_mul_frac() can't return zero. Remove the check
against zero to reflect this.

Fixes: 43b1bb4a ("clk: at91: clk-sam9x60-pll: re-factor to support plls with multiple outputs")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230227105931.2812412-1-claudiu.beznea@microchip.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>