lib: Fix timer for 32 bit

To read 64bit time in 32 bit we have to read lower & upper half
separately and 'or' them together. However, upper half time may
have changed by the time we read lower half. Thus, the resultant
64 bit time may not be accurate.

Consider lower half time value only if upper half time value has
not changed.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>