clk: at91: clk-master: check if div or pres is zero

Check if div or pres is zero before using it as argument for ffs().
In case div is zero ffs() will return 0 and thus substracting from
zero will lead to invalid values to be setup in registers.

Fixes: 7a110b91 ("clk: at91: clk-master: re-factor master clock")
Fixes: 75c88143 ("clk: at91: clk-master: add master clock support for SAMA7G5")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20211011112719.3951784-9-claudiu.beznea@microchip.com


Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>