video: fbdev: sis: fix typos in SiS_GetModeID()

The second operand of a '&&' operator has no impact on expression
result for cases 400 and 512 in SiS_GetModeID().

Judging by the logic and the names of the variables, in both cases a
typo was made.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Rustam Subkhankulov <subkhankulov@ispras.ru>
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Helge Deller <deller@gmx.de>