rt2x00: Fix for rt2800usb for SHARED_KEY_TABLE initializations

Fixed comments about SHARED_KEY_TABLE_BASE and SHARED_KEY_MODE_BASE.
Fixed initialization loop for SHARED_KEY_MODE_ENTRY.

Based on a patch for rt2800pci from Benoit.

Signed-off-by: Benoit PAPILLAULT <benoit.papillault@free.fr>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>