wlcore: increase aggregation buffer size by one page

With 4 pages (16Kb), we can't fit 10 frames in the aggregation buffer
during iperf.  This is the optimal for the firmware.  Thus, increase
the buffer size by one page.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>