smc911x: Fix build warnings

Commit 6af1d41a "smc911x MII made available" was missing a few "const"
qualifiers.  Fix the resulting in build warnings:

smc911x.c: In function 'smc911x_initialize':
smc911x.c:297: warning: passing argument 2 of 'miiphy_register' from incompatible pointer type
smc911x.c:297: warning: passing argument 3 of 'miiphy_register' from incompatible pointer type

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Helmut Raiger <helmut.raiger@hale.at>