net: stmmac: fix timestamping configuration after suspend/resume

When stmmac_init_timestamping() is called, it clears the receive and
transmit path booleans that allow timestamps to be read. These are
never re-initialised until after userspace requests timestamping
features to be enabled.

However, our copy of the timestamp configuration is not cleared, which
means we return the old configuration to userspace when requested.
This is inconsistent. Fix this by clearing the timestamp configuration.

Fixes: d6228b7cdd6e ("net: stmmac: implement the SIOCGHWTSTAMP ioctl")
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/E1vuUu4-0000000Afea-0j9B@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>