net: ethernet: sun: remove redundant variables adv and lpa and mii_reads

Variables adv and lpa are being assigned but are never used hence they
are redundant and can be removed.  Also remove the unncessary mii_reads
too.

Cleans up clang warnings:
warning: variable 'lpa' set but not used [-Wunused-but-set-variable]
warning: variable 'adv' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>