usb: usblp: Use min_t() to improve usblp_read()

Use min_t() to improve usblp_read() and avoid calculating
'avail - usblp->readcount' twice. Use min_t(ssize_t,,) instead of min()
to avoid a signedness error.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20250829173713.56222-1-thorsten.blum@linux.dev


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>