thunderbolt: Rename shadowed variables bit to interrupt_bit and auto_clear_bit

cppcheck reports
drivers/thunderbolt/nhi.c:74:7: style: Local variable 'bit' shadows outer variable [shadowVariable]
  int bit;
      ^
drivers/thunderbolt/nhi.c:66:6: note: Shadowed declaration
 int bit = ring_interrupt_index(ring) & 31;
     ^
drivers/thunderbolt/nhi.c:74:7: note: Shadow variable
  int bit;
      ^
For readablity rename the outer to interrupt_bit and the innner
to auto_clear_bit.

Fixes: 468c49f4 ("thunderbolt: Disable interrupt auto clear for ring")
Cc: stable@vger.kernel.org
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>