clk: at91: clk-master: fix prescaler logic

clk: at91: clk-master: fix prescaler logic

When prescaler value read from register is MASTER_PRES_MAX it means
that the input clock will be divided by 3. Fix the code to reflect
this.

Fixes: 7a110b9107ed8 ("clk: at91: clk-master: re-factor master clock")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20211011112719.3951784-11-claudiu.beznea@microchip.com
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>