wcnss: Add API to read WLAN RX buffer count

WLAN driver's RX buffer count is a configurable value; WCNSS platform
driver will expose an API to read this configurable value from device
tree. Using a smaller coount of this buffer will reduce the memory
usage of WLAN driver.

Change-Id: Ida3da258ec1efdc16d37e50f69de8248fd6a26f9
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>