x86/sev: Fix operator precedence in GHCB_MSR_VMPL_REQ_LEVEL macro

The GHCB_MSR_VMPL_REQ_LEVEL macro lacked parentheses around the bitmask
expression, causing the shift operation to bind too early. As a result,
when requesting VMPL1 (e.g., GHCB_MSR_VMPL_REQ_LEVEL(1)), incorrect
values such as 0x000000016 were generated instead of the intended
0x100000016 (the requested VMPL level is specified in GHCBData[39:32]).

Fix the precedence issue by grouping the masked value before applying
the shift.

  [ bp: Massage commit message. ]

Fixes: 34ff6590 ("x86/sev: Use kernel provided SVSM Calling Areas")
Signed-off-by: Seongman Lee <augustus92@kaist.ac.kr>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250511092329.12680-1-cloudlee1719@gmail.com