Bluetooth: btintel_pcie: validate TX skb length in send_sync

btintel_pcie_prepare_tx() copies skb->len bytes into a fixed
BTINTEL_PCIE_BUFFER_SIZE (4096) DMA slot via an unchecked memcpy.
Oversized packets are currently rejected only in
btintel_pcie_send_frame(); any future caller of
btintel_pcie_send_sync() would silently overflow the DMA buffer.

Add the bounds check in btintel_pcie_send_sync() itself, right
before skb_push() and the DMA copy.

Assisted-by: Copilot:claude-sonnet-5 code-review code-generation
Fixes: 6e65a09f9275 ("Bluetooth: btintel_pcie: Add *setup* function to download firmware")
Signed-off-by: Chandrashekar Devegowda <chandrashekar.devegowda@intel.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>