sound: Simplify chn_init()

• Remove unnecessary CHN_[UN]LOCKs.
• Improve device_printf() messages.
• Remove redundant checks.
• Remove 0 assignments, since the channel is allocated with M_ZERO.
• Re-organize sections in a more coherent way.
• Remove "out1" label, just return NULL directly.
• Rename "out2" to "fail" and simplify logic.
• Do not check the return value of dsp_unit2name(), as it is guaranteed not to fail (we pass a valid channel type).

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D45985

(cherry picked from commit 5b209e153b58515c0315f5902e18ecd7d75ecd2a)