drivers/net/sfc: fix sparse warnings: Should it be static?

Impact: Include the header file.  If CONFIG_SFC_MTD is not defined
then the functions 'efx_mtd_remove' and 'efx_mtd_rename' are defined
'static inline' with an empty function body in the header file, but
the the whole file mtd.c is not built in this case.

Fix this sparse warnings:
  drivers/net/sfc/mtd.c:204:6: warning: symbol 'efx_mtd_remove' was not declared. Should it be static?
  drivers/net/sfc/mtd.c:221:6: warning: symbol 'efx_mtd_rename' was not declared. Should it be static?
  drivers/net/sfc/mtd.c:230:5: warning: symbol 'efx_mtd_probe' was not declared. Should it be static?

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>