arm64: signal: Fix some under-bracketed UAPI macros

A few SME-related sigcontext UAPI macros leave an argument
unprotected from misparsing during macro expansion.

Add parentheses around references to macro arguments where
appropriate.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Fixes: ee072cf7 ("arm64/sme: Implement signal handling for ZT")
Fixes: 39782210 ("arm64/sme: Implement ZA signal handling")
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240729152005.289844-1-Dave.Martin@arm.com


Signed-off-by: Will Deacon <will@kernel.org>