net: stmmac: remove write-only mac->pmt

mac_device_info->pmt is only ever written, nothing reads it. Remove
this struct member.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1umsf5-008vKQ-DT@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>