efi_loader: efi_net: let efi_net_set_dp properly update the device path

efi_loader: efi_net: let efi_net_set_dp properly update the device path

This commit fixes an use after free introduced in Commit e55a4acb54
(" efi_loader: net: set EFI bootdevice device path to HTTP when loaded
from wget"). The logic in efi_net_set_dp is reworked so that when the
function is invoked it not only changes the value of the static variable
net_dp (this is how the function was implemented in e55a4acb54) but also
updates the protocol interface of the device path protocol in case efi
has started.

Fixes: e55a4acb54e8 ("efi_loader: net: set EFI bootdevice device path to HTTP when loaded from wget")
Signed-off-by: Adriano Cordova <adriano.cordova@canonical.com>