powerpc64/powernv: Avoid type promotion

There is a type promotion that transform count = -1 into a unsigned int causing
the default TCE SEG SIZE not being returned on a Boston POWER9 machine.

This machine does not have the 'ibm,supported-tce-sizes' entries, thus, count
is set to -1, and the function continue to execute instead of returning.

Reviewed by: jhibbits, wma
Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D15763