rt2x00: Only strip preamble bit in rt2400pci

Only rt2400pci can have the preamble bit set in the PLCP value,
for all other drivers it should not be cleared since that will
conflict with the plcp values for OFDM rates.

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