udlfb: Correct sub-optimal resolution selection.

The situation in which the problem occurred was with a Plugable UGA-2K-A
connected to a Samsung EX2220X display. The driver indicates that
1920x1080 is a valid mode (the first mode available, in fact), but
proceeds to set the framebuffer size to 1600x1200.

The patch corrects what seems to be a logic error, regarding unsetting
the FB_MISC_1ST_DETAIL flag, if the first (top/best) mode is invalid.
The existing code unset the flag if ANY mode was invalid.

Signed-off-by: William Katsak <william.katsak@alcatel-lucent.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>