checkpatch: use $String consistently

String detection where a source line with a string constant is converted
can either have an X or a space.

Some of the string detection regexes do not allow the space character, but
there is a handy $String variable that does.

Convert the remaining uses of string detection regexes to use the $String
variable to reduce possible false negatives.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>