oeqa/selftest/gcc: Fix kex exchange identification error

oeqa/selftest/gcc: Fix kex exchange identification error

while runnig oe-selftest for gcc, some of the testcases that need to be run on qemu
are not running due to below failures:
- kex_exchange_identification: read: Connection reset by peer^M
   Connection reset by 192.168.7.2 port 22^M
   ERROR: Couldn't create remote directory /tmp/runtest.3549814 on ssh

To resolve kex exchange identification error increased the MaxStartups.

Signed-off-by: Harish Sadineni <Harish.Sadineni@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>