ath5k: add debugfs file frameerrors

add a debugfs file to see different RX and TX errors as reported in our status
descriptors. this can help to diagnose driver problems.

statistics can be cleared by writing 'clear' into the frameerrors file.

example:

# cat /sys/kernel/debug/ath5k/phy0/frameerrors
RX
---------------------
CRC     27      (11%)
PHY     3       (1%)
FIFO    0       (0%)
decrypt 0       (0%)
MIC     0       (0%)
process 0       (0%)
jumbo   0       (0%)
[RX all 245]

TX
---------------------
retry   2       (9%)
FIFO    0       (0%)
filter  0       (0%)
[TX all 21]

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>