net: stmmac: dwmac-stm32: Fix Mhz to MHz

Trivial, fix up the comments using 'Mhz' to 'MHz'.
No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Christophe Roullier <christophe.roullier@foss.st.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>