spi: au1550_spi: improve pio transfer mode

Improve PIO transfer mode of au1550 spi controller by continuing of spi
transfer, instead of aborting transfer when transmit underflow interrupt
occurrs.

Verified by oscilloscope that the spi clock pauses on trasmit underflow,
so transfer continuation is perfectly valid even though au1550 datasheet
says that on tx underflow zeroes will be transfered.

Also make some error messages more specific.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Jan Nikitenko <jan.nikitenko@gmail.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>