x86: reduce redundancy in tsc_[gs]et_info()

x86: reduce redundancy in tsc_[gs]et_info()

- some of the case statements are effectively or mostly special cases
  of others, so there's no good reason not to share the code
- in the "get" function, a variable can be made case-wide instead of
  having multiple instance of it (and those even with a pointless
  initializer)
- minor formatting adjustments

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>