OvmfPkg/BaseMemEncryptSevLib: clean up upper-case / lower-case in DEBUGs

OvmfPkg/BaseMemEncryptSevLib: clean up upper-case / lower-case in DEBUGs

Debug messages that start as natural (English) language phrases (after the
debug prefix) should uniformly begin with lower-case or upper-case. In
SetMemoryEncDec() we have a mixture now. Stick with lower-case.
(Upper-case is better for full sentences that also end with punctuation.)

Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>