ManageabilityPkg: fix/cleanup request size check in SsifWriteRequest

From the IPMI v2.0 (April 21, 2015 E7 Markup) specification,
Table 22-12, Get System Interface Capabilities Command:

"Input message size in bytes. (1 based.)"
...
"A BMC that supports multi-part Start and End would return
 a value from 33 to 64. A BMC that supports multi-part with Middle
 transactions would return a value from 65 to 255."

Yet the comment in the existing code describes this as being a
counter of the number of middle packets, with Start and End packets
counting outside of that. This seems very incorrect to me.

Address this, and simplify the code, by using the already existing
global variable mMaxRequestSize. Update the debug error message
text to reflect the functional change.

Signed-off-by: Leif Lindholm <leif.lindholm@oss.qualcomm.com>