staging/mei: fix check for allocating host client id

MEI_CLIENTS_MAX is 255 and host_client_id is u8 therefore
for check to work we need to first assign the return value
of find_first_zero_bit to unsigned long variable

Fix warning

drivers/staging/mei/main.c: In function mei_open
drivers/staging/mei/main.c:260:2: warning: comparison is always false due to limited range of data type

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