staging/easycap: easycap_probe : rename usb variables to common names

1. rename usb variables to more common names

struct usb_device *pusb_device -> usbdev
struct usb_host_interface *pusb_host_interface -> alt
struct usb_endpoint_descriptor *pepd ->
struct usb_interface_descriptor *pusb_interface_descriptor -> interface;

2. use usb_altnum_to_altsetting to access alternative settings

Cc: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>