usb: musb: ux500: fix 'musbid' undeclared error in ux500_remove()

commit 65b3d52d
(usb: musb: add musb_ida for multi instance support)
used musbid in ux500_remove() but nerver declared it.

I found this in x86_64 platform, but not sure whether
this is a error on the correct ARCH.

$ make drivers/usb/musb/ux500.o
make[1]: Nothing to be done for `all'.
make[1]: Nothing to be done for `relocs'.
  CHK     include/generated/uapi/linux/version.h
  CHK     include/generated/utsrelease.h
  UPD     include/generated/utsrelease.h
  CALL    scripts/checksyscalls.sh
  CC      drivers/usb/musb/ux500.o
drivers/usb/musb/ux500.c: In function 'ux500_probe':
drivers/usb/musb/ux500.c:78:2: error: 'musbid' undeclared (first use in this function)
drivers/usb/musb/ux500.c:78:2: note: each undeclared identifier is reported only once for each function it appears in
make[1]: *** [drivers/usb/musb/ux500.o] Error 1
make: *** [drivers/usb/musb/ux500.o] Error 2

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>