checkpatch: improve MACRO_ARG_PRECEDENCE test

It is possible for a multiple line macro definition to have a false positive
report when an argument is used on a line after a continuation \.

This line might have a leading '+' as the initial character that could be
confused by checkpatch as an operator.

Avoid the leading character on multiple line macro definitions.

Link: http://lkml.kernel.org/r/60229d13399f9b6509db5a32e30d4c16951a60cd.1473836073.git.joe@perches.com


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