monitor: use signed value for factor and offset

The code was using size_t for factor and offset. For some fans the
offset could be negative, so it needs to use signed types.
This commit changes the offset type to int64_t.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: I7443b00cd9f55ff0acb2fcc077bf2f5ba2b74395
3 files changed