lan743x: make functions lan743x_csr_read and lan743x_csr_read static

Functions lan743x_csr_read and lan743x_csr_read are local to the source
and do not need to be in global scope, so make them static.

Cleans up sparse warning:
drivers/net/ethernet/microchip/lan743x_main.c:56:5: warning: symbol
lan743x_csr_read' was not declared. Should it be static?
drivers/net/ethernet/microchip/lan743x_main.c:61:6: warning: symbol
'lan743x_csr_write' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>