ide: clear interrupt on command write

Not known to fix any bug, but I couldn't help but notice that ATA
specifies that writing to this register should clear an interrupt.

ATA7: Section 5.3.3 (Command register - Effect)
ATA6: Section 7.4.4 (Command register - Effect)
ATA5: Section 7.4.4 (Command register - Effect)
ATA4: Section 7.4.4 (Command register - Effect)
ATA3: Section 5.2.2 (Command register)

Other editions: try searching for the phrase "Writing this register".

Signed-off-by: John Snow <jsnow@redhat.com>