spi: Add static keyword to spi_slave_select()

Adding the 'static' keyword to the inline routine spi_save_select() fixes
the build error when CONFIG_COMPILER_OPT="-O0 -g"

Change-Id: I5234d3d6690ccda3dbd419e13f06c2651d125167
Work-by: Mike Hirst <michael.hirst@windriver.com>
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>