wil6210: fix temperature debugfs

[ Upstream commit 6d9eb7ebae3d7e951bc0999235ae7028eb4cae4f ]

For negative temperatures, "temp" debugfs is showing wrong values.
Use signed types so proper calculations is done for sub zero
temperatures.

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>