Blackfin Serial driver: Fix bug - serial driver in PIO mode cant handle input very quickly

Output as many bytes as possible in PIO tx handler.
This reduce the number of tx interrupts and shorten the delay to handle
rx interrupt. So, rx overrun disappears.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>