staging: axis-fifo: Correct handling of tx_fifo_depth for size validation

staging: axis-fifo: Correct handling of tx_fifo_depth for size validation

Remove erroneous subtraction of 4 from the total FIFO depth read from
device tree. The stored depth is for checking against total capacity,
not initial vacancy. This prevented writes near the FIFO's full size.

The check performed just before data transfer, which uses live reads of
the TDFV register to determine current vacancy, correctly handles the
initial Depth - 4 hardware state and subsequent FIFO fullness.

Fixes: 4a965c5f89de ("staging: add driver for Xilinx AXI-Stream FIFO v4.1 IP core")
Cc: stable@vger.kernel.org
Signed-off-by: Gabriel Shahrouzi <gshahrouzi@gmail.com>
Link: https://lore.kernel.org/r/20250419012937.674924-1-gshahrouzi@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>