pwm: keembay: Fix build failure with -Os

The driver used this construct:

	#define KMB_PWM_LEADIN_MASK             GENMASK(30, 0)

	static inline void keembay_pwm_update_bits(struct keembay_pwm *priv, u32 mask,
						   u32 val, u32 offset)
	{
		u32 buff = readl(priv->base + offset);

		buff = u32_replace_bits(buff, val, mask);
		writel(buff, priv->base + offset);
	}

	...
	keembay_pwm_update_bits(priv, KMB_PWM_LEADIN_MASK, 0,
					KMB_PWM_LEADIN_OFFSET(pwm->hwpwm));

With CONFIG_CC_OPTIMIZE_FOR_SIZE the compiler (here: gcc 10.2.0) this
triggers:

	In file included from /home/uwe/gsrc/linux/drivers/pwm/pwm-keembay.c:16:
	In function ‘field_multiplier’,
	    inlined from ‘keembay_pwm_update_bits’ at /home/uwe/gsrc/linux/include/linux/bitfield.h:124:17:
	/home/uwe/gsrc/linux/include/linux/bitfield.h:119:3: error: call to ‘__bad_mask’ declared with attribute error: bad bitfield mask
	  119 |   __bad_mask();
	      |   ^~~~~~~~~~~~
	In function ‘field_multiplier’,
	    inlined from ‘keembay_pwm_update_bits’ at /home/uwe/gsrc/linux/include/linux/bitfield.h:154:1:
	/home/uwe/gsrc/linux/include/linux/bitfield.h:119:3: error: call to ‘__bad_mask’ declared with attribute error: bad bitfield mask
	  119 |   __bad_mask();
	      |   ^~~~~~~~~~~~

The compiler doesn't seem to be able to notice that with field being
0x3ffffff the expression

	if ((field | (field - 1)) & ((field | (field - 1)) + 1))
		__bad_mask();

can be optimized away.

So use __always_inline and document the problem in a comment to fix
this.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Vijayakannan Ayyathurai <vijayakannan.ayyathurai@intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>