gianfar: Fix setup of RX time stamping

Previously the RCTRL_TS_ENABLE bit was set unconditionally. However, if
the RCTRL_TS_ENABLE is set without TMR_CTRL[TE], the driver does not work
properly on some boards (Anton had problems with the MPC8313ERDB and
MPC8568EMDS).

With this patch the bit will only be set if requested from user space
with the SIOCSHWTSTAMP ioctl command, meaning that time stamping is
disabled during normal operation. Users who are not interested in time
stamps will not experience problems with buggy CPU revisions or
performance drops any more.

The setting of TMR_CTRL[TE] is still up to the user. This is considered
safe because users wanting HW timestamps must initialize the eTSEC clock
first anyway, e.g. with the recently submitted PTP clock driver.

Signed-off-by: Manfred Rudigier <manfred.rudigier@omicron.at>
Reviewed-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>