pwm: linux/pwm.h: fix Excess kernel-doc description warning

Remove the @pwm: line to prevent the kernel-doc warning:

include/linux/pwm.h:87: warning: Excess struct member 'pwm' description in 'pwm_device'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc:  <linux-pwm@vger.kernel.org>
Fixes: f3e25e68 ("pwm: Drop unused member "pwm" from struct pwm_device")
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>