mtd: rawnand: use longest matching pattern

Sometimes the exec_op parser does not choose the optimal pattern if
multiple patterns with optional elements are available. Since the stack
automatically splits operations in multiple exec_op calls, a non-optimal
pattern gets broken up into multiple calls. E.g. an OOB read using the
vf610 driver:
  nand: executing subop:
  nand:     ->CMD      [0x00]
  nand:     ->ADDR     [5 cyc: 00 08 ea 94 02]
  nand:     ->CMD      [0x30]
  nand:     ->WAITRDY  [max 200000 ms]
  nand:       DATA_IN  [64 B]
  nand: executing subop:
  nand:       CMD      [0x00]
  nand:       ADDR     [5 cyc: 00 08 ea 94 02]
  nand:       CMD      [0x30]
  nand:       WAITRDY  [max 200000 ms]
  nand:     ->DATA_IN  [64 B]

However, the vf610 driver has a pattern which can execute the complete
command in a single go...

This patch makes sure that the longest matching pattern is chosen
instead of the first (potentially only partial) match. With this
change the vf610 reads the OOB in a single exec_op call:
  nand: executing subop:
  nand:     ->CMD      [0x00]
  nand:     ->ADDR     [5 cyc: 00 08 c0 1d 00]
  nand:     ->CMD      [0x30]
  nand:     ->WAITRDY  [max 200000 ms]
  nand:     ->DATA_IN  [64 B]

Reported-by: Sascha Hauer <s.hauer@pengutronix.de>
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>