parisc/sticon: fix reverse colors

sticon_build_attr() checked the reverse argument and flipped
background and foreground color, but returned the non-reverse
value afterwards. Fix this and also add two local variables
for foreground and background color to make the code easier
to read.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>