rt2x00: rt2800lib: use rt2x00_has_cap_* helpers

Use the appropriate helper functions instead of
directly accessing the rt2x00dev->cap_flags field
to check device capability flags.

This improves readability of the code a bit.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>