mei: bus: suppress sign-compare warnings

Comparison between signed and unsigned warnings
and associated type promotion may cause error
condition not be detected.
The type promotion issue in mei bus was addressed by two patches:
commit b40b3e93 ("mei: bus: type promotion bug in mei_nfc_if_version()")
commit cf1ed2c5 ("mei: bus: type promotion bug in mei_fwver()")
Now it is possible to suppress the warning, by adding proper
casting to move out of radar.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>