[LLC]: Fix compiler warnings introduced by TX window scaling changes.

Noticed by Olaf Hering.

The comparisons want a u8 here (the data type on the left-hand branch
is a u8 structure member, and the constant on the right-hand branch is
"~((u8) 128)"), but C turns it into an integer so we get:

net/llc/llc_c_ac.c: In function `llc_conn_ac_inc_npta_value':
net/llc/llc_c_ac.c:998: warning: comparison is always true due to limited range of data type
net/llc/llc_c_ac.c:999: warning: large integer implicitly truncated to unsigned type

Fix this up by explicitly recasting the right-hand branch constant
into a "u8" once more.

Signed-off-by: David S. Miller <davem@davemloft.net>