p54usb: Fix to prevent SKB memory allocation errors with 4K page size

On x86_64 architecture with 4K page size and SLUB debugging enabled, stress
testing on p54usb has resulted in skb allocation failures of O(1) and extreme
page fragmentation. Reducing rx_mtu fixes this problem by reducing the size of
all receive skb allocations to be of O(0). This change does not impact
performance in any way.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>