staging: ft1000: Always extract device number from netdev name.

Using gCardIndex as global variable was not correctly used because
for every plugged device was set to 0 in init_ft1000_netdev.

Remove global definition. Use it locally and for every plugged
device extract number from net device name.

Signed-off-by: Marek Belisko <marek.belisko@open-nandra.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>