fbdev: omapfb: Use sysfs_emit_at() to simplify code
This file already uses sysfs_emit(). So be consistent and also use
sysfs_emit_at().
Moreover, size is always < PAGE_SIZE because scnprintf() (and now
sysfs_emit_at()) returns the number of characters written not including the
trailing '\0'. So some tests can be removed.
This slightly simplifies the code and makes it more readable.