x86/efi: tidy switch statement and address MISRA violation

x86/efi: tidy switch statement and address MISRA violation

Refactor the first clauses so that a violation of
MISRA C Rule 16.2 is resolved (a switch label, "default" in this
case, should be immediately enclosed in the compound statement
of the switch). Note that the switch clause ending with the pseudo
keyword "fallthrough" is an allowed exception to Rule 16.3.

Convert fallthrough comments in other clauses to the pseudo-keyword
while at it.

No functional change.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>