OMAP: DSS2: Replace strncmp() with sysfs_streq() in overlay_manager_store()

The function overlay_manager_store currently fails if the sysfs input is a
prefix string of an existing overlay manager name. This occurs because strncmp
compares the two strings only till the length of the input sysfs string. So a
sysfs input "lcd" will match manager name "lcd2" which is incorrect behavior.

The use of sysfs_streq here will prevent this false positive match to occur.

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