rt2x00: Fix VGC lower bound initialization

When the EEPROM_BBPTUNE_VGC word is valid, we should
override EEPROM_BBPTUNE_VGCLOWER field with the BBP
value.

And we should _not_ do that when EEPROM_BBPTUNE_R17 is valid.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>