thunderbolt: debugfs: Fix sideband write size check

sb_regs_write() looks up the matching sideband register entry before
validating the number of bytes to write.

However, the size check uses sb_regs->size, which is the size of the
first entry in the register table, instead of the matched entry. This
rejects valid writes to larger sideband registers such as USB4_SB_DEBUG
or USB4_SB_DATA.

Use the matched register entry for the size check.

Signed-off-by: Xu Rao <raoxu@uniontech.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>