rndis_host: reduce MTU instead of refusing to talk to devices with low max packet size

This patch makes the host RNDIS driver talk to RNDIS devices with an MTU
less than 1.5k, instead of refusing to talk to such a device.

Signed-Off-by: Thomas Sailer <t.sailer@alumni.ethz.ch>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>

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Hi Jeff,
are you the right person to send this to?
Nobody else seems to be wanting to forward this to Linus...

Thanks,
Tom

Signed-off-by: Jeff Garzik <jeff@garzik.org>