tools/atmelimage: add const qualifier to fix compiler warning

tools/atmelimage: add const qualifier to fix compiler warning

More strict checks in GCC 15 expose a new warning:
tools/atmelimage.c: In function ‘atmel_find_pmecc_parameter_in_token’:
tools/atmelimage.c:64:31: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
64 |                         param = strstr(token, "=");
   |                               ^
cc1: all warnings being treated as errors

Add 'const' qualifier to variable 'param' to prevent build failing
due to -Werror.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>