staging: wilc1000: refactor mgmt_tx to fix line over 80 chars

Refactor mgmt_tx() to fix line over 80 characters issue. Split the
function to avoid the checkpatch.pl warning. Returning the same error
code in case of memory allocation failure.

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