firmware: turris-mox-rwtm: fix reply status decoding function

firmware: turris-mox-rwtm: fix reply status decoding function

The status decoding function mox_get_status() currently contains an
incorrect check: if the error status is not MBOX_STS_SUCCESS, it always
returns -EIO, so the comparison to MBOX_STS_FAIL is never executed and
we don't get the actual error code sent by the firmware.

Fix this.

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Fixes: 389711b37493 ("firmware: Add Turris Mox rWTM firmware driver")
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>