net: ethernet: stmmac: drop unused variable in stm32mp1_set_mode()

Building with W=1 (cf.scripts/Makefile.extrawarn) outputs:
warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

Drop the unused 'ret' variable.

Signed-off-by: Christophe Roullier <christophe.roullier@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>