[AVR,committed]: Set case-values-threshold to 7

[Georg-Johann Lay]

http://gcc.gnu.org/viewcvs?view=revision&revision=188336

Revision 179760 changed casesi so that the libgcc code to read from
the lookup table by means of __tablejump2__ is used unconditionally,
i.e. does no more depend on -mcall-prologues.

Consequently, casesi costs no more depend on -mcall-prologues
since r179769 and are cheaper.

This patch adjusts avr_case_values_threshold and set it to 7.

Johann

	* config/avr/avr.c (avr_case_values_threshold): Return 7.