PM / devfreq: Fix typo in DFSO_DOWNDIFFERENTIAL macro name

PM / devfreq: Fix typo in DFSO_DOWNDIFFERENTIAL macro name

Correct the spelling error in the DFSO_DOWNDIFFERENTIAL macro
definition and update the corresponding variable assignment.

The macro was previously misspelled as DFSO_DOWNDIFFERENCTIAL.
This change ensures consistent and correct spelling throughout
the simpleondemand governor implementation.

Signed-off-by: Riwen Lu <luriwen@kylinos.cn>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://patchwork.kernel.org/project/linux-pm/patch/20251118032339.2799230-1-luriwen@kylinos.cn/