rndis_host: fix transfer size negotiation

This patch should resolve a problem that's troubled support for
some RNDIS peripherals.  It seems to have boiled down to using a
variable to establish transfer size limits before it was assigned,
which caused those devices to fallback to a default "jumbogram"
mode we don't support.  Fix by assigning it earlier for RNDIS.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
[ cleanups ]
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>