ice: replace "fallthrough" comments with fallthrough reserved word

"fallthrough" comments are used in switch case statements to explicitly
indicate the code is intended to fall through to the following statement.
Different variants of "fallthough" are acceptable, e.g. "fall through",
"fallthrough", "Fall-through".  The GCC compiler has an optional warning
(-Wimplicit-fallthrough[=n]) to warn when such a comment is not present;
the default version of which is enabled when compiling the Linux kernel.

There have been recent discussions in kernel mailing lists regarding
replacing non-standardized "fallthrough" comments with the pseudo-reserved
word 'fallthrough' which will be defined as __attribute__ ((fallthrough))
for versions of gcc that support it (i.e. gcc 7 and newer) or as a nop
for versions that do not.  Replace "fallthrough" comments with fallthrough
reserved word.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>