OMAP2: Fix dss_54m_fck enable_bit

Fix an incorrect enable_bit for the OMAP2 clock dss_54m_fck; prevents
the LCD backlight from lighting up on 2430 SDP.  Problem noted by Kevin
Hilman <khilman@mvista.com>; debugging assistance from Khasim Syed Mohammed
<x0khasim@ti.com> and Richard Woodruff <r-woodruff2@ti.com>.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>