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.