disas/m68k: Avoid unintended sign extension in get_field()

In get_field(), we take an 'unsigned char' value and shift it left,
which implicitly promotes it to 'signed int', before ORing it into an
'unsigned long' type.  If 'unsigned long' is 64 bits then this will
result in a sign extension and the top 32 bits of the result will be
1s.  Add explicit casts to unsigned long before shifting to prevent
this.

(Spotted by Coverity, CID 715697.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 1488556233-31246-4-git-send-email-peter.maydell@linaro.org
