net: tcp: Use an uint8_t for retry timeout instead of an uin32_t

Rename the variable to `retry_timeout_shift`, and shift-right the value
each time there's a timeout.  This saves some memory in that structure
by using the holes left due to alignment.

Change-Id: I18f45d00ecc434a588758a8d331921db902f4419
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>