switchtec: Make struct event_regs static

The structure event_regs is local to the source and does not need to be in
global scope, so make it static.

Cleans up sparse warning:

  symbol 'event_regs' was not declared. Should it be static

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Logan Gunthorpe <logang@deltatee.com>