staging: wilc1000: fix null checks on wilc

staging: wilc1000: fix null checks on wilc

Currently the pointer wilc is being null checked several times
and yet not checked for the final workqueue flush and destroy
(which can lead to a null pointer dereference if wilc is null);
these missing null checks were overlooked in an earlier core
refactoring commit.

Clean up the code by checking wilc at the start and bailing out
early if it is null allowing the subsequent null checks to be
removed, this also fixes the potential null pointer deferences
on the workqueue flush and destroy calls.

Detected by CoverityScan, CID#1473305 ("Dereference after null check")

Fixes: 11b0aa847921 ("staging: wilc1000: refactor code to move initilization in wilc_netdev_init()")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>