ARM: 5893/1: SPI AMBA PL022: Limit TX FIFO fills

Added logic to cap TX FIFO fill size based on current free RX
FIFO entries instead of TX status flags. This is to prevent
an issue with RX FIFO overflows.

Signed-off-by: Kevin Wells <kevin.wells@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>