mtd: Use kasprintf() instead of fixed buffer formatting

Using "%4.4X" in the calculation of the buffer size is misleading, as
the format string literal has no relation to the actual size needed.
Hence this is fragile w.r.t. future modification.

As this is not a hot path, fix this by replacing the formatting in a
fixed buffer by kasprintf().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>