net: Set default NET_NBUF_RX_COUNT to 4.

Older default value of 2 is too small for even simple uses, for
example enabling just CONFIG_NETWORKING, CONFIG_NET_IPV4,
CONFIG_NET_SLIP_TAP makes a Zephyr application hang on start up
(apparently, waiting to allocate more buffers). While value of 3
is enough to cover the above options, set new default to 4 to
accommodate configurations with IPv4/IPv6/UDP/TCP enabled.

While the new default slightly increases RAM footprint, it's
apparently better for users who are short of it to optimize it
explicitly, rather than potentially make every Zephyr beginner
wonder why enabling networking breaks applications in non-obvious
way.

Change-Id: I76e83517f0d914ce616a930c3c48ee5c52567b88
Jira: ZEP-1619
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>