wl18xx: fix a bug in wl->num_rx_desc initialization

wl->num_rx_desc was mistakenly initialized with WL18XX_NUM_TX_DESCRIPTORS
but it should use WL18XX_NUM_RX_DESCRIPTORS instead.

This bug was passed unnoticed because currently both RX and TX descriptors
are initialized to the same value (32).

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