nfp: correct return codes when msleep gets interrupted

msleep_interruptible() returns time left to wait, not error
code.  Return ERESTARTSYS when interrupted.

While at it correct a comment and make the polling a bit
more aggressive.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>