net: gianfar_ptp: Rename FS bit to FIPERST

FS is a global symbol used by the x86 32-bit architecture, fixes builds
re-definitions:

>> drivers/net/ethernet/freescale/gianfar_ptp.c:75:0: warning: "FS"
>> redefined
    #define FS                    (1<<28) /* FIPER start indication */

   In file included from arch/x86/include/uapi/asm/ptrace.h:5:0,
                    from arch/x86/include/asm/ptrace.h:6,
                    from arch/x86/include/asm/math_emu.h:4,
                    from arch/x86/include/asm/processor.h:11,
                    from include/linux/mutex.h:19,
                    from include/linux/kernfs.h:13,
                    from include/linux/sysfs.h:15,
                    from include/linux/kobject.h:21,
                    from include/linux/device.h:17,
                    from
drivers/net/ethernet/freescale/gianfar_ptp.c:23:
   arch/x86/include/uapi/asm/ptrace-abi.h:15:0: note: this is the
location of the previous definition
    #define FS 9

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>