spi: spi-mem: fix coverity report CID 537478

spi: spi-mem: fix coverity report CID 537478

Coverity finds a potential integer overflow in the following code:

  ncycles += ((op->data.nbytes * 8) / op->data.buswidth) / (op->data.dtr ? 2 : 1);

A quick analysis shows that the only caller of the suspicious code is the
spinand_select_op_variant() function from the drivers/mtd/nand/spi/core.c
file.

According to the code the value of op->data.nbytes is equal to

  nanddev_per_page_oobsize(nand) + nanddev_page_size(nand)

Therefore it's maximum value a bit larger than 4Kb (I never seen flashes
with page size large than 4Kb). So op->data.nbytes always fits within
13 bits. As result an overflow will never happen.

Anyway it's better fix an issue to eliminate the error message.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>