NTB: ntb_transport: Fix too small buffer for debugfs_name

NTB: ntb_transport: Fix too small buffer for debugfs_name

The buffer used for "qp%d" was only 4 bytes, which truncates names like
"qp10" to "qp1" and causes multiple queues to share the same directory.

Enlarge the buffer and use sizeof() to avoid truncation.

Fixes: fce8a7bb5b4b ("PCI-Express Non-Transparent Bridge Support")
Cc: <stable@vger.kernel.org> # v3.9+
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Jon Mason <jdmason@kudzu.us>