rt2x00: add rt2x00_has_cap_* helpers

The rt2x00 code directly accesses the 'cap_flags'
field of 'struct rt2x00_dev' when checking presence
of a given capability flag. The direct access needs
long expressions which lowers readability of the code.

Add a few helper functions which can be used to test
device capabilities without directly accessing the
cap_flags filed.

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