hwmon: (npcm750-pwm-fan) Remove an unused field in struct npcm7xx_cooling_device

In "struct npcm7xx_cooling_device", the 'fan_clk_freq' field is unused.
Remove it.

Found with cppcheck, unusedStructMember.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/74eee8aa739f94b8c6425ce3e37a427ca92243ea.1714505655.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Guenter Roeck <linux@roeck-us.net>