m68k: net: Fix unused variable in mcfmii.c

The following warning was produced, fix it:

mcfmii.c: In function 'mcffec_miiphy_write':
mcfmii.c:318:8: warning: variable 'rdreg' set but not used [-Wunused-but-set-variable]

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: "Jin Zhengxiong-R64188" <R64188@freescale.com>
Cc: Jason Jin <jason.jin@freescale.com>