HID: core: remove unneeded assignment in hid_process_report()

Commit bebcc522

 ("HID: core: for input reports, process the usages by
priority list") split the iteration into two distinct loops in
hid_process_report().

After this change, the variable field is only used while iterating in the
second loop and the assignment of values to this variable in the first loop
is simply not needed.

Remove the unneeded assignment during retrieval. No functional change and
no change in the resulting object code.

This was discovered as a dead store with clang-analyzer.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Tom Rix <trix@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>