Input: max77693-haptic - fix potential overflow

Expression haptic->pwm_dev->period * haptic->magnitude is of type
'unsigned int' and may overflow. We need to convert one of the operands
to u64 before multiplying, instead of casting result (potentially
overflown) to u64.

Reported by Coverity: CID 1248753

Acked-by : Jaewon Kim <jaewon02.kim@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>