OMAP: DSS2: Fix: Read correct bit in dispc_enable_alpha_blending()

Read correct DISPC_CONFIG bit when the channel is DIGIT, remove extra line
at the end of the function.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>