Fix xiao_m0 spi clock used in spi_master

[task-jp]

Summary

Use proper clock in spi_master

35e3a1a#diff-50cf0af7e1c3cc57008455475eef99c880de47307bbf5b685c24ca1f0f985a0cL136 changed sercom0 -> sercom4 accidentally.

Checklist

• CHANGELOG.md for the BSP or HAL updated
• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced (see CI or check locally)

If Adding a new Board

• Board CI added to crates.json
• Board is properly following "Tier 2" conventions, unless otherwise decided to be "Tier 1"

If Adding a new cargo feature to the HAL

• Feature is added to the test matrix for applicable boards / PACs in crates.json