stmmac: dwmac-loongson: change loongson_dwmac_driver from global to static

Smatch reports this issue
dwmac-loongson.c:208:19: warning: symbol
  'loongson_dwmac_driver' was not declared.
  Should it be static?

loongson_dwmac_driver is only used in dwmac-loongson.c.
File scope variables used only in one file should
be static. Change loongson_dwmac_driver's
storage-class-specifier from global to static.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>