Staging: brcm80211: sys: wlc_ampdu: fix some comparison warnings

Use min_t() instead of min() in some places to make the comparison
explicit and resolve some compiler warnings.

Cc: Brett Rudley <brudley@broadcom.com>
Cc: Henry Ptasinski <henryp@broadcom.com>
Cc: Nohee Ko <noheek@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>