ipmi_si: fix potential integer overflow on large shift

Shifting unsigned char b by an int type can lead to sign-extension
overflow. For example, if b is 0xff and the shift is 24, then top
bit is sign-extended so the final value passed to writeq has all
the upper 32 bits set.  Fix this by casting b to a 64 bit unsigned
before the shift.

Detected by CoverityScan, CID#1465246 ("Unintended sign extension")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>