ixp4xx: drop the jumbo frame support

ixp4xx: drop the jumbo frame support

Current jumbo frame support code allocates rxbuffers of the maximum size
supported by the hardware (~14KB). This happens at the device open time
even if the configured MTU is lower (e.g. if it configured to standard
1500 bytes). Such behavior effectivly prevents interface start on boards
with a low ammount of RAM (e.g. WRT300N v2), since the kernel simly can
not allocates of ~0.8MB (14KB x 64).

So remove jumbo frame support for now.

Reported-by: Nerijus Baliunas <nerijus@users.sourceforge.net>
Tested-by: Nerijus Baliunas <nerijus@users.sourceforge.net>
Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>