Merge branch 'smsc-w-1-warning-fixes'

Andrew Lunn says:

====================
smsc W=1 warning fixes

Fixup various W=1 warnings, and then add COMPILE_TEST support, which
explains why these where missed on the previous pass.

v2:
Use while (0)
Rework buffer alignment to make it clearer

v3:
Access the length from the hardware and Use __always_unused to tell the
compiler we want to discard the value.
====================

Link: https://lore.kernel.org/r/20201110030248.1480413-1-andrew@lunn.ch
Signed-off-by: Jakub Kicinski <kuba@kernel.org>