spi/spi-pl022: Don't allocate more sg than required.

In routine configure_dma(), if transfer->len = PAGE_SIZE, then pages is one more
than required. While leads to one more sg getting allocated.

This is wrong. Correct this to allocate correct number of sg.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>