clk: stm32f4: fix an uninitialized variable

clk: stm32f4: fix an uninitialized variable

The variable s, used by pr_debug() to print the mnemonic of the modulation
depth in use, was not initialized. Fix the output by addressing the correct
mnemonic.

Fixes: 65b3516dbe50 ("clk: stm32f4: support spread spectrum clock generation")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/77355eb9-19b3-46e5-a003-c21c0fae5bcd@stanley.mountain
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Link: https://lore.kernel.org/r/20250124111711.1051436-1-dario.binacchi@amarulasolutions.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>