usb: udc: lpc32xx: remove set but not used 3 variables

Fixes gcc '-Wunused-but-set-variable' warning:
drivers/usb/gadget/udc/lpc32xx_udc.c: In function ‘udc_protocol_cmd_r’:
drivers/usb/gadget/udc/lpc32xx_udc.c:744:6: warning: variable ‘tmp’ set but not used [-Wunused-but-set-variable]

drivers/usb/gadget/udc/lpc32xx_udc.c: In function ‘udc_handle_dma_ep’:
drivers/usb/gadget/udc/lpc32xx_udc.c:1994:14: warning: variable ‘epstatus’ set but not used [-Wunused-but-set-variable]

drivers/usb/gadget/udc/lpc32xx_udc.c: In function ‘udc_handle_ep0_setup’:
drivers/usb/gadget/udc/lpc32xx_udc.c:2200:22: warning: variable ‘wLength’ set but not used [-Wunused-but-set-variable]

It is not used since commit 90fccb52 ("usb: gadget: Gadget directory cleanup - group UDC drivers")

Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>