Input: elan_i2c - ignore signals when finishing updating firmware

Use wait_for_completion_timeout() instead of
wait_for_completion_interruptible_timeout() to avoid stray signals ruining
firmware update. Our timeout is only 300 msec so we are fine simply letting
it expire in case device misbehaves.

Signed-off-by: KT Liao <kt.liao@emc.com.tw>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>