rtlwifi: Initialize rate_init member of struct rate_control_ops

This partially reverts commit 44ba9736.

rate_control_rate_init assumes the rate_init member of
struct rate_control_ops is not NULL therefore not initializing it leads to
an oops as soon the driver succesfully associates to an AP.

The removal of rate_update from 44ba9736
is ok because rate_update is checked for NULL before being
called.

Signed-off-by: Catalin Iacob <iacobcatalin@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>