SMSC95XX updates for packet alignment and turbo mode (V2)

[Ben Dooks]

This is the new seris of SMSC95XX patches to deal with the issues we
found when working on this driver for a client. The new series has been
tested on an Raspberry Pi3 B.

Changes since v1:
 - Change memcpy to use {get,put}_unaligned_le32() calls
 - Merge tx fixups
 - Added rx_turbo attribute