usb: gadget: udc: dummy_hcd: Repair misspelled function argument 'dummy_hcd'

usb: gadget: udc: dummy_hcd: Repair misspelled function argument 'dummy_hcd'

An attempt was made to document the functions in 'dummy_hcd',
but a simple spelling mistake was made.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/gadget/udc/dummy_hcd.c:1597: warning: Function parameter or member 'dum_hcd' not described in 'handle_control_request'
 drivers/usb/gadget/udc/dummy_hcd.c:1597: warning: Excess function parameter 'dum' description in 'handle_control_request'

Cc: Felipe Balbi <balbi@kernel.org>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200703174148.2749969-31-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>