mwifiex: replace netif_carrier_on/off by netif_device_attach/dettach

Driver is doing netif_carrier_off during suspend, which will set
the IFF_LOWER_UP flag to 0. As a result certain applications
will think this as a real carrier down and behave accordingly.
This will cause issues like loss of IP address, for example. To
fix this use netif_device_dettach during suspend.

Fixes: 0026b32d ('mwifiex: fix Tx timeout issue during suspend test')

Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>