staging: r8712u: Reduce maximum receive buffer size to a more sensible value

The current value for the maximum receive buffer size is 30720, which is
too large. For long-running systems, memory fragmentation may make it
difficult to obtain the buffers of O(2) needed for aggregation. Buffers
of O(3) are even worse, particularly when not needed. The new size is set
to 9100, which will allow aggregation.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>