iopoll: change uS to us in comments

S is the symbol for Siemens but we're referring to seconds in this
file. Change uS to us to avoid confusion.

This is based on feedback from upstream:
http://lists.infradead.org/pipermail/linux-arm-kernel/2014-June/268025.html



Change-Id: Ic148f9d3b84758f0d74f3edc45097067f8d88c49
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>