gatt: Fix assuming writes to CCC will always contain 2 bytes

gatt: Fix assuming writes to CCC will always contain 2 bytes

The spec actually allows writing just 1 byte:

BLUETOOTH CORE SPECIFICATION Version 5.1 | Vol 3, Part F
page 2320:

  'If the attribute value has a fixed length and the Attribute Value
   parameter length is less than or equal to the length of the attribute
   value, the octets of the attribute value parameter length shall be
   written; all other octets in this attribute value shall be
   unchanged.'

Change-Id: I845531fbd44262a5ab29c2353f63c16ed98480bc
Signed-off-by: himanshu <h.himanshu@samsung.com>