[ Upstream commit 6571ebce112a21ec9be68ef2f53b96fcd41fd81b ]

If fill_level was not zero and status was not BUSY,
result of "tx_prod - tx_cons - inuse" might be zero.
Subtracting 1 unconditionally results invalid negative return value
on this case.
Make sure not to return an negative value.
Signed-off-by: NTomonori Sakita <tomonori.sakita@sord.co.jp>
Signed-off-by: NAtsushi Nemoto <atsushi.nemoto@sord.co.jp>
Reviewed-by: NDalon L Westergreen <dalon.westergreen@linux.intel.com>
Acked-by: NThor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NSasha Levin <sashal@kernel.org>