staging: wilc1000: avoid use of static variable 'inactive_time'

Avoided the use of static variable 'inactive_time' and move it as part of
'sta_inactive_t' structure.

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>