usb: gadget: r8a66597: fix unused-but-set-variable warnings

This patch fixes the following (W=1) warnings:

drivers/usb/gadget/udc/r8a66597-udc.c: In function ‘r8a66597_irq’:
drivers/usb/gadget/udc/r8a66597-udc.c:1468:15: warning: variable ‘nrdyenb’ set but not used [-Wunused-but-set-variable]
  u16 brdyenb, nrdyenb, bempenb;
               ^
drivers/usb/gadget/udc/r8a66597-udc.c:1467:15: warning: variable ‘nrdysts’ set but not used [-Wunused-but-set-variable]
  u16 brdysts, nrdysts, bempsts;
               ^

In doing so, it removes calls to r8a66597_read function which does I/O
with the device, but I hope the reads don’t have any side effects that
are needed.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>