ath9k: Check for errors when reading SREV register

Right now, if an error is encountered during the SREV register
read (i.e. an EIO in ath9k_regread()), that error code gets
passed all the way to __ath9k_hw_init(), where it is visible
during the "Chip rev not supported" message.

    ath9k_htc 1-1.4:1.0: ath9k_htc: HTC initialized with 33 credits
    ath: phy2: Mac Chip Rev 0x0f.3 is not supported by this driver
    ath: phy2: Unable to initialize hardware; initialization status: -95
    ath: phy2: Unable to initialize hardware; initialization status: -95
    ath9k_htc: Failed to initialize the device

Check for -EIO explicitly in ath9k_hw_read_revisions() and return
a boolean based on the success of the operation. Check for that in
__ath9k_hw_init() and abort with a more debugging-friendly message
if reading the revisions wasn't successful.

    ath9k_htc 1-1.4:1.0: ath9k_htc: HTC initialized with 33 credits
    ath: phy2: Failed to read SREV register
    ath: phy2: Could not read hardware revision
    ath: phy2: Unable to initialize hardware; initialization status: -95
    ath: phy2: Unable to initialize hardware; initialization status: -95
    ath9k_htc: Failed to initialize the device

This helps when debugging by directly showing the first point of
failure and it could prevent possible errors if a 0x0f.3 revision
is ever supported.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>