net: fec: Define indexes as 'unsigned int'

net: fec: Define indexes as 'unsigned int'

Fix the following warnings that happen when building with W=1 option:

drivers/net/ethernet/freescale/fec.c: In function 'fec_enet_free_buffers':
drivers/net/ethernet/freescale/fec.c:1337:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/net/ethernet/freescale/fec.c: In function 'fec_enet_alloc_buffers':
drivers/net/ethernet/freescale/fec.c:1361:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
drivers/net/ethernet/freescale/fec.c: In function 'fec_enet_init':
drivers/net/ethernet/freescale/fec.c:1631:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Fabio Estevam <[email protected]>
Signed-off-by: David S. Miller <[email protected]>