ipmi: msghandler: Add and use pr_fmt and dev_fmt, remove PFX

Standardize the prefixing of output messages using the pr_fmt and dev_fmt
mechanisms instead of a separate #define PFX

Miscellanea:

o Because this message prefix is very long, use a non-standard define
  of #define pr_fmt(fmt) "%s" fmt, "IPMI message handler: "
  which removes ~170 bytes of object code in an x86-64 defconfig with ipmi
  (with even more object code reduction on 32 bit compilations)

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>