mtd: r852: make ecc_reg 32-bit in r852_ecc_correct

r852_ecc_correct() reads a 32-bit register into a 16-bit variable,
ecc_reg, but this variable is later used as if it was larger.  This is
reported by clang when building the kernel with many warnings:

    drivers/mtd/nand/r852.c:512:11: error: shift count >= width of type
    [-Werror,-Wshift-count-overflow]
                    ecc_reg >>= 16;
                            ^   ~~
Fix this by making ecc_reg 32-bit, like the return type of
r852_read_reg_dword().

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>