staging: wfx: make hif_scan() usage clearer

hif_scan() return max number of jiffies to wait before the completion
indication. However, if this value is negative, an error has occurred.

Reword the code to reflect that behavior.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200115135338.14374-3-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>