net/sfc/base: decorate libefx internal extern functions

net/sfc/base: decorate libefx internal extern functions

The decorator may be used in the future to instruct linker
to put it into dedicated sections or hide.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Reviewed-by: Mark Spender <mspender@xilinx.com>
Reviewed-by: Richard Houldsworth <rhouldsw@xilinx.com>