rt2800pci: make Kconfig help entry more helpful

Document known issues with the driver to aid distribution makers,
users and developers in making informed decisions instead of wasting
their time needlessly.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>