Make PL011 UART to wait on putc only when TX FIFO is full

Instead of waiting for empty TX FIFO it is more reasonable to
block on full FIFO. As soon as FIFO slot is free the character
can be transmitted.
In case of TX FIFO disabled, TXFF bit indicates that transmit
register is not empty.

Obtained from: Semihalf
Reviewed by: andrew, emaste
Sponsored by: The FreeBSD Foundation