Ethernut 5: fix build error

Fix build error for ethernut5 board due to prototype change
for function board_mmc_getcd().
ethernut5.c:238: error: conflicting types for 'board_mmc_getcd'
u-boot/include/mmc.h:318: note: previous declaration of 'board_mmc_getcd'
was here
make[2]: *** [ethernut5.o] Error 1

Signed-off-by: Prabhakar Lad <prabhakar.csengg@gmail.com>
Cc: Tim Schendekehl <tim.schendekehl@egnite.de>
Cc: Thierry Reding <thierry.reding@avionic-design.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: egnite GmbH <info@egnite.de>