mtd: onenand: Fix uninitialized retlen in do_otp_read()

The function do_otp_read() does not set the output parameter *retlen,
which is expected to contain the number of bytes actually read.
As a result, in onenand_otp_walk(), the tmp_retlen variable remains
uninitialized after calling do_otp_walk() and used to change
the values of the buf, len and retlen variables.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 49dc08ee ("[MTD] [OneNAND] fix numerous races")
Cc: stable@vger.kernel.org
Signed-off-by: Ivan Stepchenko <sid@itb.spb.ru>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>