mtd: rawnand: fsmc: Fix unchecked return value in fsmc_read_page_hwecc

Check return value of nand_read_data_op.

Notice that, currently, all instances of nand_read_data_op() are
being checked, with the exception of two of them in marvell_nand
driver, in which the caller function explicitly returns 0 every
time.

Also, notice that I moved the declaration of *ret* to the top of
fsmc_read_page_hwecc().

Addresses-Coverity-ID: 1471968 ("Unchecked return value")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>