i2c: xiic: defer RX_FULL until all trailing bytes are in FIFO

For the normal path of xiic_smbus_block_read_setup() -- the trailing
bytes all fit in one Rx FIFO fill -- RFD was programmed two below the
byte count, which fires the RX_FULL interrupt while the last byte is
still in flight. xiic_read_rx() then lands in its bytes_rem == 1 branch
and sets NACK on a byte still on the wire, truncating the read.

Without PEC this is harmless: the truncated byte is the dummy one the
caller never looks at. With PEC enabled it is the PEC byte itself, and
i2c_smbus_check_pec() fails the transfer with -EBADMSG.

Raise the threshold by one so RX_FULL fires only once every remaining
byte is already buffered. That routes the drain through
xiic_read_rx()'s bytes_rem == 0 path, which reads everything out and
emits the stop cleanly. The only change for the non-PEC case is that
the controller waits one extra byte-time before servicing the
interrupt.

rfd_set stays inside the 4 bits of XIIC_RFD_REG_OFFSET: this branch is
only reached when rxmsg_len + pec_len <= IIC_RX_FIFO_DEPTH, so the
value is at most IIC_RX_FIFO_DEPTH - 1.

Fixes: e4c1ff772e1a ("i2c: xiic: Add smbus_block_read functionality")
Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
Cc: <stable@vger.kernel.org> # v6.3+
Acked-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://patch.msgid.link/20260924-i2c-xiic-v7-2-df7e752332ef@nexthop.ai

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