media: atmel-isi: use 'time_left' variable with wait_for_completion_timeout()

There is a confusing pattern in the kernel to use a variable named
'timeout' to store the result of wait_for_completion_timeout() causing
patterns like:

        timeout = wait_for_completion_timeout(...)
        if (!timeout) return -ETIMEDOUT;

with all kinds of permutations. Use 'time_left' as a variable to make the
code self explaining.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>