octeontx2-pf: fix "UBSAN: shift-out-of-bounds error"

In the Linux kernel, the following vulnerability has been resolved:

octeontx2-pf: fix "UBSAN: shift-out-of-bounds error"

This patch ensures that the RX ring size (rx_pending) is not

set below the permitted length. This avoids UBSAN

shift-out-of-bounds errors when users passes small or zero

ring sizes via ethtool -G.