qtnfmac: modify Rx descriptors queue setup

Rx descriptors queue length is hardware specific. Current common default
value is no more than an accident. So move Rx descriptor queue setup to
platform PCIe backend in the same way as it is already done for Tx.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>