MT762x HW, except for MT7628, supports frame length up to 2048
(maximum length on GDM), so allow setting MTU up to 2030.

Also set the default frame length to the hardware default 1518.
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210125042046.5599-1-dqfext@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>