arm64: sysreg: Fix unprotected macro argmuent in write_sysreg

write_sysreg() may misparse the value argument because it is used
without parentheses to protect it.

This patch adds the ( ) in order to avoid any surprises.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
[will: same change to write_sysreg_s]
Signed-off-by: Will Deacon <will.deacon@arm.com>