rt2x00: Increase REGISTER_BUSY_COUNT

For some hardware the REGISTER_BUSY_COUNT isn't sufficient,
increase the REGISTER_BUSY_COUNT to 100 to catch most
devices which have more problems with accessing the registers.

For normal operating devices nothing would change as they will
exit the loop early anyway.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>