net: Extra '_get' in declaration of arch_get_platform_mac_address

In commit c7f5d105

 ("net: Add eth_platform_get_mac_address() helper."),
two declarations were added:

  int eth_platform_get_mac_address(struct device *dev, u8 *mac_addr);
  unsigned char *arch_get_platform_get_mac_address(void);

An extra '_get' was introduced in arch_get_platform_get_mac_address, remove
it. Fix compile warning using W=1:

  CC      net/ethernet/eth.o
net/ethernet/eth.c:523:24: warning: no previous prototype for ‘arch_get_platform_mac_address’ [-Wmissing-prototypes]
 unsigned char * __weak arch_get_platform_mac_address(void)
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  AR      net/ethernet/built-in.o

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>