net: ethernet: freescale: fix false-positive string overflow warning

While compile-testing on arm64 with gcc-8.1, I ran into a build diagnostic:

drivers/net/ethernet/freescale/fec_main.c: In function 'fec_probe':
drivers/net/ethernet/freescale/fec_main.c:3517:25: error: '%d' directive writing between 1 and 10 bytes into a region of size 5 [-Werror=format-overflow=]
   sprintf(irq_name, "int%d", i);
                         ^~
drivers/net/ethernet/freescale/fec_main.c:3517:21: note: directive argument in the range [0, 2147483646]
   sprintf(irq_name, "int%d", i);
                     ^~~~~~~
drivers/net/ethernet/freescale/fec_main.c:3517:3: note: 'sprintf' output between 5 and 14 bytes into a destination of size 8
   sprintf(irq_name, "int%d", i);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It appears this has never shown on ppc32 or arm32 for an unknown reason, but
now gcc fails to identify that the 'irq_cnt' loop index has an upper bound
of 3, and instead uses a bogus range.

To work around the warning, this changes the sprintf to snprintf with the
correct buffer length.

Fixes: 78cc6e7e

 ("net: ethernet: freescale: Allow FEC with COMPILE_TEST")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>