mwifiex: rename usb driver name registerring to usb core

Both libertas USB driver and mwifiex_usb driver are registerring
with name 'usb8xxx'. The following conflict happens while trying
to load both drivers.

[6.211307] Error: Driver 'usb8xxx' is already registered...
[6.217261] mwifiex_usb: Driver register failed!

Fix it by renaming mwifiex_usb driver's name.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>