checkpatch: add #define foo "string" long line exception

There are #defines with long string constants like:
	#define foo "some really long string > 80 columns"
Add a long line exception for them.

Miscellanea:

Use the $String variable for slightly better readability

Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Madalin-Cristian Bucur <madalin.bucur@freescale.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>