staging: emxx_udc: fix the build error

Fix below build error:

reproduce:
  wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross

 -O ~/bin/make.cross
  chmod +x ~/bin/make.cross
  git checkout 9239d88f
  # save the attached .config to linux build tree
  make.cross ARCH=arm

All error/warnings:

   In file included from include/linux/seqlock.h:35:0,
                    from include/linux/time.h:5,
                    from include/linux/stat.h:18,
                    from include/linux/module.h:10,
                    from drivers/staging/emxx_udc/emxx_udc.c:22:
   drivers/staging/emxx_udc/emxx_udc.c: In function 'nbu2ss_gad_set_selfpowered':
>> drivers/staging/emxx_udc/emxx_udc.c:3129:21: error: 'udc' undeclared (first use in this function)
     spin_lock_irqsave(&udc->lock, flags);
                        ^
   include/linux/spinlock.h:215:34: note: in definition of macro 'raw_spin_lock_irqsave'
      flags = _raw_spin_lock_irqsave(lock); \
                                     ^
>> drivers/staging/emxx_udc/emxx_udc.c:3129:2: note: in expansion of macro 'spin_lock_irqsave'
     spin_lock_irqsave(&udc->lock, flags);
     ^
   drivers/staging/emxx_udc/emxx_udc.c:3129:21: note: each undeclared identifier is reported only once for each function it appears in
     spin_lock_irqsave(&udc->lock, flags);
                        ^
   include/linux/spinlock.h:215:34: note: in definition of macro 'raw_spin_lock_irqsave'
      flags = _raw_spin_lock_irqsave(lock); \
                                     ^
>> drivers/staging/emxx_udc/emxx_udc.c:3129:2: note: in expansion of macro 'spin_lock_irqsave'
     spin_lock_irqsave(&udc->lock, flags);
     ^

vim +/udc +3129 drivers/staging/emxx_udc/emxx_udc.c

33aa8d45 Magnus Damm 2014-06-06  3123
33aa8d45 Magnus Damm 2014-06-06  3124  	if (pgadget == NULL) {
33aa8d45 Magnus Damm 2014-06-06  3125  		ERR("%s, bad param\n", __func__);
33aa8d45 Magnus Damm 2014-06-06  3126  		return -EINVAL;
33aa8d45 Magnus Damm 2014-06-06  3127  	}
33aa8d45 Magnus Damm 2014-06-06  3128
33aa8d45 Magnus Damm 2014-06-06 @3129  	spin_lock_irqsave(&udc->lock, flags);
9239d88f Peter Chen  2015-01-28  3130  	pgadget->is_selfpowered = (is_selfpowered != 0);
33aa8d45 Magnus Damm 2014-06-06  3131  	spin_unlock_irqrestore(&udc->lock, flags);
33aa8d45 Magnus Damm 2014-06-06  3132

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>