net: usb: sr9700: Use 'SR_' prefix for the common register macros

The commone register macors (e.g. RSR) is too commont to drivers, it may
be conflict with the architectures (e.g. xtensa, sh).

The related warnings (with allmodconfig under xtensa):

    CC [M]  drivers/net/usb/sr9700.o
  In file included from drivers/net/usb/sr9700.c:24:0:
  drivers/net/usb/sr9700.h:65:0: warning: "RSR" redefined
   #define RSR   0x06
   ^
  In file included from ./arch/xtensa/include/asm/bitops.h:22:0,
                   from include/linux/bitops.h:36,
                   from include/linux/kernel.h:10,
                   from include/linux/list.h:8,
                   from include/linux/module.h:9,
                   from drivers/net/usb/sr9700.c:13:
  ./arch/xtensa/include/asm/processor.h:190:0: note: this is the location of the previous definition
   #define RSR(v,sr) __asm__ __volatile__ ("rsr %0,"__stringify(sr) : "=a"(v));
   ^

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>