mtd: fix mtd_oobavail() incoherent returned value

mtd: fix mtd_oobavail() incoherent returned value

BugLink: https://bugs.launchpad.net/bugs/1857158
[ Upstream commit 4348433d8c0234f44adb6e12112e69343f50f0c5 ]

mtd_oobavail() returns either mtd->oovabail or mtd->oobsize. Both
values are unsigned 32-bit entities, so there is no reason to pretend
returning a signed one.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>