s390/raw3270: fix nullpointer check

Fix the following checkpatch warning:

CHECK: Comparison to NULL could be written "!rp"
+       if (rp == NULL)

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>