mtd-tests: nandbiterrs: Fix issue that just insert error at bit 7

The function insert_biterror should be designed to insert error at
the first '1' bit starting at offset byte.

But now, only bit 7 of each byte is checked, because checking mask
is always 0x80.

So, do right shift for checking mask after each checking to check
the whole 8 bits of each bytes.

Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>