alpha: Define pgprot_modify to silence tautological comparison warnings

alpha: Define pgprot_modify to silence tautological comparison warnings

Alpha's pgprot_noncached, pgprot_writecombine, and pgprot_device are
all identity macros, so the generic pgprot_modify() produces
tautological self-comparisons that GCC warns about:

  include/linux/pgtable.h:1701:25: warning: self-comparison always
  evaluates to true [-Wtautological-compare]

Since all caching attributes are no-ops on Alpha, define
pgprot_modify() to simply return newprot.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://lore.kernel.org/r/20260403150128.488513-1-mattst88@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>