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>