ath9k: Fix error check in ath9k_hw_read_revisions() for PCI devices

When the error check in ath9k_hw_read_revisions() was added, it checked for
-EIO which is what ath9k_regread() in the ath9k_htc driver uses. However,
for plain ath9k, the register read function uses ioread32(), which just
returns -1 on error. So if such a read fails, it still gets passed through
and ends up as a weird mac revision in the log output.

Fix this by changing ath9k_regread() to return -1 on error like ioread32()
does, and fix the error check to look for that instead of -EIO.

Fixes: 2f90c7e5 ("ath9k: Check for errors when reading SREV register")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210326180819.142480-1-toke@redhat.com