generic: drop support for get_port_stats() on ar8xxx

generic: drop support for get_port_stats() on ar8xxx

The implementation is not efficient on ar8xxx switches. It triggers high
CPU load and degrades device performance.

The high CPU load has been traced down to the ar8xxx_reg_wait() call in
ar8xxx_mib_op(), which has to usleep_range() till the MIB busy flag set
by the request to update the MIB counter is cleared.

This commit removes the get_port_stats() code introduced in 4d8a66d and
leaves a note for future hacker's beware.

Fixes: FS#1004
Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>