pwm: omap-dmtimer: Repair pwm_omap_dmtimer_chip's broken kerneldoc header

Argument descriptions must be prepended with a '@' to be understood
by the kerneldoc tooling/parsers/validators.

Fixes the following W=1 warning:

  drivers/pwm/pwm-omap-dmtimer.c:70: warning: Function parameter or member 'dm_timer_pdev' not described in 'pwm_omap_dmtimer_chip'

Cc: Tony Lindgren <tony@atomide.com>
Cc: Joachim Eastwood <manabian@gmail.com>
Cc: NeilBrown <neilb@suse.de>
Cc: Grant Erickson <marathon96@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Author: Lee Jones

drivers/pwm/pwm-omap-dmtimer.c

@@ -58,7 +58,7 @@
  * @mutex:		Mutex to protect pwm apply state
  * @dm_timer:		Pointer to omap dm timer.
  * @pdata:		Pointer to omap dm timer ops.
- * dm_timer_pdev:	Pointer to omap dm timer platform device
+ * @dm_timer_pdev:	Pointer to omap dm timer platform device
  */
 struct pwm_omap_dmtimer_chip {
 	struct pwm_chip chip;