Replace if-elseif-else with switch-case
to keep the code consistent which is semantically same

Switch-case is used here,
http://www.spinics.net/lists/linux-usb/msg17201.html
Making consistent at other places in usb/core

Also easier to read and maintain when USB4.0, 5.0, ... comes
Signed-off-by: Viral Mehta <viral.mehta@einfochips.com>
Tested-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>