drivers: spi: stm32h7: Use a better name for ll_func_tx_is_empty

In H7, TXP indicates when its FIFO has room for, at least, one
packet. Thus, rename ll_func_tx_is_empty as ll_func_tx_is_not_full,
to be consistent in all platforms.

Signed-off-by: Daniel Gaston Ochoa <dgastonochoa@gmail.com>