if_smsc: Add the ability to disable "turbo_mode", also called RX frame batching,
similarly to the Linux driver, by a tunable read only sysctl.

Submitted by: Oleg Sidorkin <osidorkin@gmail.com>
PR: 254884
Sponsored by: Mellanox Technologies // NVIDIA Networking

(cherry picked from commit 5a3426f453f970edda38367bea5ebf7385c3819d)