IB/hfi1: Use fallthrough pseudo-keyword

IB/hfi1: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with the
new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7-rc7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Link: https://lore.kernel.org/r/20200721133455.GA14363@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>