DynamicTablesPkg: Fix IORT node length assignment

DynamicTablesPkg: Fix IORT node length assignment

The VS2017 compiler reports 'warning C4267: 'return': conversion
from 'size_t' to 'UINT32', possible loss of data' for a number of
functions that compute the IORT node length. Similarly, it reports
warnings for IORT node length field assignments as the length
field is 16-bit wide.

This patch adds type casts at appropriate places and also implements
validations to ensure that the max width of the respective fields
is not exceeded.

This patch also fixes a typo in one of the local variable names.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Alexei Fedorov <Alexei.Fedorov@arm.com>